Ulamog's ability triggers as you cast it, and that ability resolves before the spell itself. It resolves even if that spell is countered.
Ulamog, the Infinite Gyre
Lethal damage and effects that say "destroy" won't cause a creature with indestructible to be put into the graveyard. However, a creature with indestructible can be put into the graveyard for a number of reasons. The most likely reasons are if it's sacrificed, if it's legendary and another legendary creature with the same name is controlled by the same player, or if its toughness is 0 or less.
Annihilator abilities trigger and resolve during the declare attackers step. The defending player chooses and sacrifices the required number of permanents before they declare blockers. Any creatures sacrificed this way won't be able to block.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Aleksi Briclot
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
