Thorn-Thrash Viashino

Devour 2 (As this creature enters, you may sacrifice any number of creatures. It enters with twice that many +1/+1 counters on it.)
{G}: This creature gains trample until end of turn.

Planechase Anthology

Devour 2 (As this enters the battlefield, you may sacrifice any number of creatures. This creature enters the battlefield with twice that many +1/+1 counters on it.)
{G}: Thorn-Thrash Viashino gains trample until end of turn.

Planechase 2012

Shards of Alara

Devour 2 (As this comes into play, you may sacrifice any number of creatures. This creature comes into play with twice that many +1/+1 counters on it.)
{G}: Thorn-Thrash Viashino gains trample until end of turn.

If multiple creatures with devour are entering under your control at the same time, you may use each one's devour ability. A creature you already control can be devoured by only one of them, however. (In other words, you can't sacrifice the same creature to satisfy multiple devour abilities.) All creatures devoured this way are sacrificed at the same time.

If you cast this as a spell, you choose how many and which creatures to devour as part of the resolution of that spell. (It can't be countered at this point.) The same is true of a spell or ability that lets you put a creature with devour onto the battlefield.

You may choose not to sacrifice any creatures for the Devour ability.

You may sacrifice only creatures that are already on the battlefield. If a creature with devour and another creature are entering under your control at the same time, the creature with devour can't devour that other creature. The creature with devour also can't devour itself.