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 wonâ€™t resolve and none of its effects will happen. You wonâ€™t get any Eldrazi Scions.
Sacrificing an Eldrazi Scion creature token to add to your mana pool is a mana ability. It doesnâ€™t use the stack and canâ€™t be responded to.
Eldrazi Scions are similar to Eldrazi Spawn, seen in the Zendikar block. Note that Eldrazi Scions are 1/1, not 0/1.
Eldrazi and Scion are each separate creature types. Anything that affects Eldrazi will affect these tokens, for example.