When Carrier Thrall dies, create a 1/1 colorless Eldrazi Scion creature token. It has "Sacrifice this creature: Addto your mana pool."
8/25/2015 Eldrazi Scions are similar to Eldrazi Spawn, seen in the Zendikar block. Note that Eldrazi Scions are 1/1, not 0/1.
8/25/2015 Eldrazi and Scion are each separate creature types. Anything that affects Eldrazi will affect these tokens, for example.
8/25/2015 Sacrificing an Eldrazi Scion creature token to add to your mana pool is a mana ability. It doesn’t use the stack and can’t be responded to.
8/25/2015 Some instants and sorceries that create Eldrazi Scions require targets. If all targets for such a spell have become illegal by the time that spell tries to resolve, the spell will be countered and none of its effects will happen. You won’t get any Eldrazi Scions.