The List

Arcbound Slith

Whenever Arcbound Slith deals combat damage to a player, put a +1/+1 counter on it.
Modular 1 (This creature enters with a +1/+1 counter on it. When it dies, you may put its +1/+1 counters on target artifact creature.)

Planechase

Whenever Arcbound Slith deals combat damage to a player, put a +1/+1 counter on it.
Modular 1 (This enters the battlefield with a +1/+1 counter on it. When it's put into a graveyard, you may put its +1/+1 counters on target artifact creature.)

Darksteel

Whenever Arcbound Slith deals combat damage to a player, put a +1/+1 counter on it.
Modular 1 (This comes into play with a +1/+1 counter on it. When it's put into a graveyard, you may put its +1/+1 counters on target artifact creature.)