Boneyard GnawerG
Creature - Mongoose
Hexproof
Boneyard Gnawer gets +1/+1 for each permanent card in opponents' graveyards.
1/1
Swiftstrike Assailant R
Creature - Human Soldier
First Strike
You may cast Swiftstrike Assailant as though it had flash if a creature an opponent controls became tapped this turn.
3/1
Vigoreaper B
Creature - Horror
Deathtouch
If a spell or ability would cause a creature an opponent controls to gain power or toughness, it causes that creature to lose that much power or toughness instead.
2/1
Disrupting Tern U
Creature - Bird Wizard
Flying
Discard a land card: counter target spell unless its controller pays 1.
1/1
Bloodcrazed Stalwart W
Creature - Human Soldier
Vigilance
Whenever a creature an opponent controls dies, Bloodcrazed Stalwart deals 3 damage to that creature's controller.
2/2
There are fun and color pie wise are perfect, a couple of them seem too good. Vigoreaper is already a very powerful hatebear that flat out shuts certain decks down (Weenie, Tokens, etc.) and it having deathtouch makes it that much more versatile.
I like Disrupting Tern a lot and seems like it would be a blast to play in a Life of the Loam control deck.
You still worded it as a trigger. You have to word it like Qasali Ambusher. It's impossible to only be able to cast when a creature becomes tapped, there has to be a condition that is true that allows it to be cast.
Swiftstrike Assailant is incredibly broken with the "without paying its mana cost" clause. a 3/1 first strike for 2 is already rare worthy material compared to Firedrinker Satyr, and giving it situational flash compounds the deal. Mono red decks would splash white without question just for this card without that clause. But with it, it can also be run in any deck ever, because your opponent attacking you is way too easy to make happen. Seriously, you could be running mono black control and have 4 of these in your deck, because it basically reads as:
Swiftspear 0
Instant
Cast Swiftspear only if a creature is attacking you.
Put a 3/1 creature token with first strike onto the battlefield.
Except yours actually has more upside than that card. It would easily break Modern, let alone Standard. I don't know much about Legacy, but I assume it would still be playable there.
The others I would say are fine, with the exception of Bloodcrazed Stalwart. That is way too much life loss to staple onto a 2cmc card. Because of the way things work, if you play End Hostilities with this on the field, it will still trigger for each dying creature. It could easily hit your opponent for 15 damage on turn 5. Considering its attached to a very solid body for its cost, 1 damage would be fine, or 2 damage if it were a symmetrical effect like Eidolon of the Great Revel. (And this is not even mentioning its playable in the same colors as Swiftspear.)
Creature - Mongoose
Hexproof
Boneyard Gnawer gets +1/+1 for each permanent card in opponents' graveyards.
1/1
Swiftstrike Assailant R
Creature - Human Soldier
First Strike
You may cast Swiftstrike Assailant as though it had flash if a creature an opponent controls became tapped this turn.
3/1
Vigoreaper B
Creature - Horror
Deathtouch
If a spell or ability would cause a creature an opponent controls to gain power or toughness, it causes that creature to lose that much power or toughness instead.
2/1
Disrupting Tern U
Creature - Bird Wizard
Flying
Discard a land card: counter target spell unless its controller pays 1.
1/1
Bloodcrazed Stalwart W
Creature - Human Soldier
Vigilance
Whenever a creature an opponent controls dies, Bloodcrazed Stalwart deals 3 damage to that creature's controller.
2/2
I like Disrupting Tern a lot and seems like it would be a blast to play in a Life of the Loam control deck.
UBRKess, Dissident MageUBR - Controlling Dissidents
GRhonas the IndomitableG - Indomitable Four Drops
WUBOloro, Ageless AsceticWUB - Loot & Renanimate
Swiftspear 0
Instant
Cast Swiftspear only if a creature is attacking you.
Put a 3/1 creature token with first strike onto the battlefield.
Except yours actually has more upside than that card. It would easily break Modern, let alone Standard. I don't know much about Legacy, but I assume it would still be playable there.
The others I would say are fine, with the exception of Bloodcrazed Stalwart. That is way too much life loss to staple onto a 2cmc card. Because of the way things work, if you play End Hostilities with this on the field, it will still trigger for each dying creature. It could easily hit your opponent for 15 damage on turn 5. Considering its attached to a very solid body for its cost, 1 damage would be fine, or 2 damage if it were a symmetrical effect like Eidolon of the Great Revel. (And this is not even mentioning its playable in the same colors as Swiftspear.)
Avant Block: Avant -- Stormfront