True, and thinking about it... large token creatures are often blue (illusions and such) so perhaps the Baron could be 4UB with a black/blue twist on his abilities. Something like "Whenever a creature dealt damage by a Vampire you control this turn is put into a graveyard choose one - put a 4/4 Black Vampire creature token with flying into play; or look at the top card of your library. You may put that card on the bottom of your library."?
I like how the mechanics tie in with the flavor: you can't stop this enchantment or the targeted creature any more than you can stop the tides. However, targeted unblockabiltiy is very, very powerful. The same ability on Ethereal Usher costs twice as much to get into play, although in that case you get a creature plus transmute and this card costs an additional mana to activate (more or less). But EU is removed fairly easily whereas this is next to irremovable. I think this would be an excellent card costed at 3UU.
Since kertrats asked for feedback on the concept and not just the card, I'll say that I like the concept. The drawback is significant and makes the creature almost worthless but if the player dies before the end of the turn the the drawback doesn't trigger. In the early game it will block and kill just about anything and would make an interesting combo with a card like:
P/T Twiddle
W
Enchantment - Aura
Enchant creature
W: Enchanted creature gets -2/+2 until end of turn.
The mechanic itself it a color pie challenge. Giving your opponent life isn't really defined in any color but if I had to pick one I'd say it belongs in green or maybe white, although there is Grollub. Personally I'd make this card green, but if you keep it black I'd consider going maybe 5/5 plus evasion to make it a more useful finisher.
Seeing as how the Baron is not blue, it's rather out of flavor to give Castle Sengir a blue ability. And less face it, the Baron and his kin are all about flavor. That said, while I like the original theme of the Baron just fine, I LOVE your version! Not only does it capture the flavor of the Baron's influence slowly growing via his vampire army beautifully, but it also is a mechanical variation of a vampire ability that hasn't been done before. The only flaw is perhaps that the tokens are more powerful than the average vampire card. I'd also consider leaving his cost as 3BBB. Yes, it makes him less playable but the Baron is about as black as black ever gets so he makes most sense in a monoblack deck.
Avyier, Who Saves the Day: Undercosted. Even with the Legendary drawback 2/1 flying with a special ability for UU is a bit over blue's curve. I've not quite decided if the special ability is too cheap or not since it does have the "at end of turn" clause, but the presence of Obtainer makes for a pretty darn cheap draw engine.
Source of Knowledge: Again, undercosted. Archivist was printed in the most broken block ever and it cost twice as much as this card and was easier to remove. The risk of a two-for-one loss as an aura is negated by the fact that you'll be able to immediately use the ability to replace the card 99% of the time.
Recognition: Again... undercosted. Counsel of the Soratami is a playable card and is two cards for 2U. The difference between one colored mana and two in the casting cost is significant but whether it is a fair trade depends on the effect. 1UU and 3U is a balanced trade for a counterspell. UU and 2U for card drawing is not.
Complicated Thoughts: Cool effect but the wording is unclear.
Loss of Sanity: Remove the random clause. The card is strong, the effect is good, but random discard is generally unfun. Forcing an opponent to choose between pitching two potentially useful cards or taking significant damage is twisted and evil and fantastic!
Insanity: The discussion on this one is still open so... even without Dark Ritual the spell is still overpowered. X spells with discard walk a very thin line between being unplayable and being broken.
Cursed Animal: Undercosted. 5/5 trample for three mana is a bomb in any format. Yukora, the Prisoner is playable and costs one more mana without trample. This card's drawback would almost balance it out if it didn't also have recurssion built in.
Shadow Bringer: In my opinion, this is the worst offfender. It's out of flavor and undercosted. Haste is almost exclusively a red ability. It is given to black extremely rarely and always paired with graveyard recursion. Of the 38 cards in standard that have or give haste, only two are black: Goryo's Vengeance and Akuta, Born of Ash. Additionally, Smogsteed Rider has a similar ability to give all your other creatures fear, and it is only a 2/2 for one less mana with no other ability.
Mark of Pain: Simply broken. It's a fair tool in most creature matchups, but wiping out every 1-toughness creature your opponent controls whenever you want for two mana is way too cheap.
Empty Mind: All 1 cc discard spells in the game either give your opponent the choice or miss basic land.
The Sin and It's Punishment: Interesting effect but it's much worse for your opponent and too powerful for one mana.
Relinquish: This card is either black or white, probably white.
Fabric of Wisdom: Undercosted. Card drawing, life gain, 2/2 body with tutoring for GG.
First off, Quest is a very powerful tutoring ability and green has traditionally been weak at tutoring. If it was restricted to simple creatures, like Llanowar Sentinal was, I could see it. But on sorceries and such it's a bit out of flavor and too strong for green.
A quick comment about card costing. I've noticed quite a few cards in the list that are improved versions of existing cards. This is a noble pursuit but remember the original cards balanced costs and abilities the way they did for a reason. Maybe their final card was too far below the curve to be playable, but a more powerful version has a high chance of being broken, and it's better to design a bad card than a broken one. I'll stop being preachy now and get to individual card comments.
Barlet, Immortal Warrior - Uncounterability is an ability exclusive to red and green. There is only one card in standard with uncounterability that is not red or green and it's a blue counterspell. (I will be referring to standard a bit because it is the best guide for determining the current power curve and color pie configuration.) It's ok to bend the rules of the color pie a bit, but you should have a purpose for doing so, otherwise you hurt the game in general.
Final Fight: White rarely gives more than +1/+1 or +0/+2 to all it's creatures; green is the color that does big boosts. To compare, Charge Across the Araba is the only white card in standard that can give a bonus that big to all your creatures and it costs an additional mana and has the sweep cost. Sacrificing your creatures is a bit more white than green, so monogreen doesn't quite make sense either. This card makes the most sense as white/green.
Total Ruination: I see that your intention was to swap mana for a drawback on Wrath, but I don't think any playable Wrath effect should cost less than four; three mana is just too cheap for such a big effect. In the right deck the drawback is actually a bonus and most decks don't run much regeneration. As a comparison, look at Harsh Mercy. I think the approriate cost would actually be 2WW or even 3WW.
Royal Decree: Even at sorcery speed, this is a very powerful effect for one mana as it essentially removes any risk from an alpha strike since blocked attackers become available to block next turn. Additionally, you can pick and choose between favorable and unfavoarbale blocking assignments. I think it would be vastly more interesting and appropriately costed if the effect was not optional, which would force a great deal of strategy as to what matchups the defender chooses and which creatures the attacking player will protect or trade.
Fractured: White is moving away from artifact destruction. A little remains in standard but the card pool for artifact destruction is overwhelmingly green and red. This card isn't 100% out of flavor, but it should serve a clear purpose and not be a standard inclusion in your set.
Overall good creativity and some interesting ideas, but there are two problems that often occur among your cards: out-of-color abilities and undercosting. If my post doesn't get sucked into oblivion due to registration issues, I'll comment on specific cards for you.
P/T Twiddle
W
Enchantment - Aura
Enchant creature
W: Enchanted creature gets -2/+2 until end of turn.
The mechanic itself it a color pie challenge. Giving your opponent life isn't really defined in any color but if I had to pick one I'd say it belongs in green or maybe white, although there is Grollub. Personally I'd make this card green, but if you keep it black I'd consider going maybe 5/5 plus evasion to make it a more useful finisher.
Source of Knowledge: Again, undercosted. Archivist was printed in the most broken block ever and it cost twice as much as this card and was easier to remove. The risk of a two-for-one loss as an aura is negated by the fact that you'll be able to immediately use the ability to replace the card 99% of the time.
Recognition: Again... undercosted. Counsel of the Soratami is a playable card and is two cards for 2U. The difference between one colored mana and two in the casting cost is significant but whether it is a fair trade depends on the effect. 1UU and 3U is a balanced trade for a counterspell. UU and 2U for card drawing is not.
Complicated Thoughts: Cool effect but the wording is unclear.
Loss of Sanity: Remove the random clause. The card is strong, the effect is good, but random discard is generally unfun. Forcing an opponent to choose between pitching two potentially useful cards or taking significant damage is twisted and evil and fantastic!
Insanity: The discussion on this one is still open so... even without Dark Ritual the spell is still overpowered. X spells with discard walk a very thin line between being unplayable and being broken.
Cursed Animal: Undercosted. 5/5 trample for three mana is a bomb in any format. Yukora, the Prisoner is playable and costs one more mana without trample. This card's drawback would almost balance it out if it didn't also have recurssion built in.
Shadow Bringer: In my opinion, this is the worst offfender. It's out of flavor and undercosted. Haste is almost exclusively a red ability. It is given to black extremely rarely and always paired with graveyard recursion. Of the 38 cards in standard that have or give haste, only two are black: Goryo's Vengeance and Akuta, Born of Ash. Additionally, Smogsteed Rider has a similar ability to give all your other creatures fear, and it is only a 2/2 for one less mana with no other ability.
Mark of Pain: Simply broken. It's a fair tool in most creature matchups, but wiping out every 1-toughness creature your opponent controls whenever you want for two mana is way too cheap.
Empty Mind: All 1 cc discard spells in the game either give your opponent the choice or miss basic land.
The Sin and It's Punishment: Interesting effect but it's much worse for your opponent and too powerful for one mana.
Relinquish: This card is either black or white, probably white.
Fabric of Wisdom: Undercosted. Card drawing, life gain, 2/2 body with tutoring for GG.
First off, Quest is a very powerful tutoring ability and green has traditionally been weak at tutoring. If it was restricted to simple creatures, like Llanowar Sentinal was, I could see it. But on sorceries and such it's a bit out of flavor and too strong for green.
A quick comment about card costing. I've noticed quite a few cards in the list that are improved versions of existing cards. This is a noble pursuit but remember the original cards balanced costs and abilities the way they did for a reason. Maybe their final card was too far below the curve to be playable, but a more powerful version has a high chance of being broken, and it's better to design a bad card than a broken one. I'll stop being preachy now and get to individual card comments.
Barlet, Immortal Warrior - Uncounterability is an ability exclusive to red and green. There is only one card in standard with uncounterability that is not red or green and it's a blue counterspell. (I will be referring to standard a bit because it is the best guide for determining the current power curve and color pie configuration.) It's ok to bend the rules of the color pie a bit, but you should have a purpose for doing so, otherwise you hurt the game in general.
Final Fight: White rarely gives more than +1/+1 or +0/+2 to all it's creatures; green is the color that does big boosts. To compare, Charge Across the Araba is the only white card in standard that can give a bonus that big to all your creatures and it costs an additional mana and has the sweep cost. Sacrificing your creatures is a bit more white than green, so monogreen doesn't quite make sense either. This card makes the most sense as white/green.
Total Ruination: I see that your intention was to swap mana for a drawback on Wrath, but I don't think any playable Wrath effect should cost less than four; three mana is just too cheap for such a big effect. In the right deck the drawback is actually a bonus and most decks don't run much regeneration. As a comparison, look at Harsh Mercy. I think the approriate cost would actually be 2WW or even 3WW.
Royal Decree: Even at sorcery speed, this is a very powerful effect for one mana as it essentially removes any risk from an alpha strike since blocked attackers become available to block next turn. Additionally, you can pick and choose between favorable and unfavoarbale blocking assignments. I think it would be vastly more interesting and appropriately costed if the effect was not optional, which would force a great deal of strategy as to what matchups the defender chooses and which creatures the attacking player will protect or trade.
Fractured: White is moving away from artifact destruction. A little remains in standard but the card pool for artifact destruction is overwhelmingly green and red. This card isn't 100% out of flavor, but it should serve a clear purpose and not be a standard inclusion in your set.
More to come.