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Athreos seems anti-synergistic with all your tokens, and giving your opponent a choice is usually a bad idea. I'd cut him to max out out on Bitterblossom. Gatekeeper of Malakir probably isn't worth it in the board unless you plan on running into lots of Bogles (keep it in by all means if you do). I think Leyline of Sanctity in the board is a necessity against burn, especially since you plan on losing a lot of life to Thoughtseize and Bitterblossom. Tidehollow Sculler is probably a bit excessive given the discard suite you already have with the full playsets of Thoughtseize and Inquisition of Kozilek.
Browbeat is bad because you never get what you need. Giving your opponents a choice is basically never good, which is why punisher cards (i.e Browbeat) are almost universally bad. When you're running out of cards, your opponent will take the 5 and when you really wish you had direct damage they'll let you draw a few cards.
I'm not sure how Confidant is too slow--it's arguably the fastest and best card advantage engine in the format, and I fail to see how that is "not enough utility." It's an extra card a turn stapled onto an aggressive body. Because of how low your curve is, you won't ever be taking more than two or three off it and you will desperately need ways to refill your hand. That's exactly the issue with Browbeat--once you're out of gas, your opponent will just take the 5 and you'll be down for the count.
It's just repeatable damage and board control once you have a recursion engine. I imagine it specifically is to help against Lingering Souls tokens, with the handy ability to go to the dome when necessary.
I'd be careful throwing those words around--they're rather vague and unhelpful. Especially since you have no idea how the OP's playgroup works or what they think is acceptable, I don't think you can really talk about any "spirit of the game."
That said, I do agree that it'd be a good idea to check with your playgroup about whether or not collusion to this degree is okay. Arcum Dagsson or Hermit Druid combo decks (for the sake of illustration) are powerful enough without additional support--if you have someone else supporting them it becomes oppressive, unfair, and likely not very fair for the rest of your playgroup.
Blue Mana Battery seems bad, especially since it's so easy for you to lose all the mana you've dumped into it if it gets destroyed. If you're intent on storing mana, Calciform Pools and Sand Silos are both harder-to-remove options. It doesn't really seem like you have enough artifacts for Filigree Angel to be all that useful. Soul's Attendant doesn't do a whole lot if you're not running any ways to abuse lots of life (Serra Ascendant, Felidar Sovereign, etc.).
Some duals may help smooth out your manabase. Glacial Fortress is the cheapest option, and would do a lot of work.
To make pretty deckboxes, just type "[.deck]" before your list and "[./deck]" at the end (without the dots) with the numbers before the cardnames.
In terms of your list:
Index is just bad. It doesn't let you get rid of things you don't want, and it doesn't give you any cards right away. Serum Visions and Sleight of Hand are both much more powerful replacements.
I'm also not seeing the combo here--it might just be me, but you may also want to explain it in the OP. Regardless of what it is, I highly doubt Venser's Journal contributes much to it. The card has zero impact on the board when you drop it, and doesn't put in a huge amount of work once you do. I'd cut it.
Icy Blast also isn't a particularly impressive card. Cryptic Command can do the same thing for most likely less mana, and gives you the options to do a bunch more useful stuff.
The additional discard in the SB seems excessive, considering you're already running a full 8 discard spells mainboard. Night of Souls' Betrayal is cute, but Choke is just way better Twin hate, and you get splash damage on Patriot and Scapeshift.
Considering Trinket Mage fetches a full three quarters of your artifacts, he seems like a good inclusion, especially if you find yourself having difficulty finding combo pieces.
More Path to Exile and less Thunderous Wrath seems like a good idea. You're kind of light on unconditional spot removal, and Thunderous Wrath can be an incredibly clunky card.
Magus of the Moon also seems really lackluster when you could just be running Blood Moon maindeck. Sure, he hits for two, but most decks you'd want him out against can kill him pretty easy and runs creatures that far outclass him at every stage of the game.
Inkmoth Nexus is an additional manland that can kill pretty quickly with the right support.
Since you plan on running at least 12 colorless sources, you're going to want to try to be as color-light as possible. The alternative to that is running color-filtering things like Odyssey filterlands and Ravnica signets.
I'll third the votes to cut Demonic Tutor and Grim Harvest. I'll also second the vote to cute Soldevi Agnate, and vote to cut the two Loxodon Warhammers. All of our big guys have evasion of one sort of another, and it doesn't seem particularly important to what we want to be doing.
Sliver legion is 5 mana and even worse, all colors. This makes it extremely difficult to use if you haven't drawn the right lands.
I think Sliver Legion is an excellent curve-topper as maybe a on- or two-of. All but four of the lands in the deck produce all five colors, so I don't think there'll be much trouble casting it.
Scroll Rack would be excellent for this deck. Being able to stack the top seven cards of your deck is amazing in general, but especially when it lets you choose which free spells you want to get.
The original flashback cards had the little tombstone next to their name as a reminder. I think they figured it would be easier to distinguish in the graveyard.
This. During the Odyssey block, which was graveyard-focused, any cards with Flashback had a tombstone next to their cardname. When the transition to the modern card frame was made, it was decided that the symbol was too confusing and aesthetically unpleasing and it was dropped.
4x Bloodghast
4x Blood Artist
4x Kalastria Highborn
4x Viscera Seer
2x Athreos, God of Passage
2x Blood Baron of Vizkopa
Instant & Sorcery (14)
4x Lingering Souls
4x Path to Exile
4x Inquisition of Kozilek
2x Thoughtseize
2x Bitterblossom
2x Sorin, Solemn Visitor
Lands (22)
4x Marsh Flats
3x Godless Shrine
4x Isolated Chapel
3x Cavern of Souls
5x Swamp
3x Plains
2x Gatekeeper of Malakir
2x Relic of Progenitus
2x Tidehollow Sculler
2x Thoughtseize
2x Stony Silence
2x Surgical Extraction
2x Deathmark
1x Utter End
In terms of your list:
Athreos seems anti-synergistic with all your tokens, and giving your opponent a choice is usually a bad idea. I'd cut him to max out out on Bitterblossom. Gatekeeper of Malakir probably isn't worth it in the board unless you plan on running into lots of Bogles (keep it in by all means if you do). I think Leyline of Sanctity in the board is a necessity against burn, especially since you plan on losing a lot of life to Thoughtseize and Bitterblossom. Tidehollow Sculler is probably a bit excessive given the discard suite you already have with the full playsets of Thoughtseize and Inquisition of Kozilek.
I'm not sure how Confidant is too slow--it's arguably the fastest and best card advantage engine in the format, and I fail to see how that is "not enough utility." It's an extra card a turn stapled onto an aggressive body. Because of how low your curve is, you won't ever be taking more than two or three off it and you will desperately need ways to refill your hand. That's exactly the issue with Browbeat--once you're out of gas, your opponent will just take the 5 and you'll be down for the count.
EDIT: You underestimate Inquisition of Kozilek. Games aren't won with high-CMC bombs, they're won with Tarmogoyf and Lingering Souls and Deceiver Exarch and Arcbound Ravager.
It's just repeatable damage and board control once you have a recursion engine. I imagine it specifically is to help against Lingering Souls tokens, with the handy ability to go to the dome when necessary.
I'd be careful throwing those words around--they're rather vague and unhelpful. Especially since you have no idea how the OP's playgroup works or what they think is acceptable, I don't think you can really talk about any "spirit of the game."
That said, I do agree that it'd be a good idea to check with your playgroup about whether or not collusion to this degree is okay. Arcum Dagsson or Hermit Druid combo decks (for the sake of illustration) are powerful enough without additional support--if you have someone else supporting them it becomes oppressive, unfair, and likely not very fair for the rest of your playgroup.
Some duals may help smooth out your manabase. Glacial Fortress is the cheapest option, and would do a lot of work.
In terms of your list:
Index is just bad. It doesn't let you get rid of things you don't want, and it doesn't give you any cards right away. Serum Visions and Sleight of Hand are both much more powerful replacements.
I'm also not seeing the combo here--it might just be me, but you may also want to explain it in the OP. Regardless of what it is, I highly doubt Venser's Journal contributes much to it. The card has zero impact on the board when you drop it, and doesn't put in a huge amount of work once you do. I'd cut it.
Icy Blast also isn't a particularly impressive card. Cryptic Command can do the same thing for most likely less mana, and gives you the options to do a bunch more useful stuff.
Magus of the Moon also seems really lackluster when you could just be running Blood Moon maindeck. Sure, he hits for two, but most decks you'd want him out against can kill him pretty easy and runs creatures that far outclass him at every stage of the game.
You can Stifle Portcullis' triggered LTB ability.
Inkmoth Nexus is an additional manland that can kill pretty quickly with the right support.
Since you plan on running at least 12 colorless sources, you're going to want to try to be as color-light as possible. The alternative to that is running color-filtering things like Odyssey filterlands and Ravnica signets.
4 Priest of Yawgmoth
1 Soldevi Adnate (2 @ -1)
4 Myr Retriever
2 Royal Assassin
2 Moriok Rigger
4 Yawgmoth Demon
2 Scuttling Doom Engine
1 Steel Hellkite
4 Perilous Myr
2 Cathodion
2 Executioner's Capsule
4 Ichor Wellspring
3 Puppet Strings
2 Loxodon Warhammer (2 @-1
2 Trading Post (1 @+1)
2 Ugin's Nexus
1 Spine of Ish Sah (2 @+1)
1 Sol Ring
2 Nim Deathmantle
1 Mana Vault
2 Go for the Throat
1 Yawgmoth's Bargain
Lands: 24
13 Phyrexia's Core (2 @-1)
4 Vault of Whispers
3 Mishra's Factory
2 Urborg, Tomb of Yawgmoth
2 Buried Ruin
I think Sliver Legion is an excellent curve-topper as maybe a on- or two-of. All but four of the lands in the deck produce all five colors, so I don't think there'll be much trouble casting it.
Scroll Rack would be excellent for this deck. Being able to stack the top seven cards of your deck is amazing in general, but especially when it lets you choose which free spells you want to get.
This. During the Odyssey block, which was graveyard-focused, any cards with Flashback had a tombstone next to their cardname. When the transition to the modern card frame was made, it was decided that the symbol was too confusing and aesthetically unpleasing and it was dropped.