When does the Army enter?

So Sauron the Dark Lord is on the field which says WHENEVER an opponent casts a Spell, amass Orcs 1.

My question is when does the Orc army come in? It says whenever so I would assume it would be AFTER the cast because usually it would say ON cast when it would be happening at the same time?

Also this would be a board state action correct? Meaning it doesn't use the stack and it just happens?