Trojan Horse

Haste Echo (At the beginning of your upkeep, if this came under your control since the beginning of your last upkeep, sacrifice it unless you pay its echo cost.) When this creature enters, tap all other creatures.

You don't lose unspent green mana as steps and phases end. Omnath gets +1/+1 for each unspent green mana you have.

: Target creature gets +X/+X until end of turn, where X is the number of creatures you control.