You choose which creature you control to sacrifice as the first triggered ability resolves. Players can respond to this ability triggering, but once you choose a creature to sacrifice, it’s too late for anyone to respond.

If Smothering Abomination is the only creature you control as its first triggered ability resolves, you must sacrifice it.

Sacrificing Smothering Abomination will cause its last ability to trigger.

Cards with devoid use frames that are variations of the transparent frame traditionally used for Eldrazi. The top part of the card features some color over a background based on the texture of the hedrons that once imprisoned the Eldrazi. This coloration is intended to aid deckbuilding and game play.

A card with devoid is just colorless. It’s not colorless and the colors of mana in its mana cost.

Other cards and abilities can give a card with devoid color. If that happens, it’s just the new color, not that color and colorless.

Devoid works in all zones, not just on the battlefield.

If a card loses devoid, it will still be colorless. This is because effects that change an object’s color (like the one created by devoid) are considered before the object loses devoid.