I was playing Untap, and I had a Cumber Stone out that gave all opponent's creatures -1/0. He casted Warstorm Surge, and proceeded to drop a bunch of 5/5s with Omnath, Locus of Rage. He said my artifact's effect doesn't kick in until they are already on the battlefield, but since it's a static effect I think they would be 4/5s when they enter. Am I wrong?
Cumber Stone's effect is a continuous effect from a static ability (C.R. 604.1, 604.2), so it applies "at all times" that Cumber Stone is on the battlefield (C.R. 611.3b, 112.6). The effect is not "locked in" (C.R. 611.3a), so it can even apply to creatures your opponents control that entered the battlefield after Cumber Stone did.
Your opponent is right in that Cumber Stone doesn't apply until they are already on the field (how would they apply any sooner anyway?). However, that is good enough for Warstorm Surge. There is never a time when those creatures are on the battlefield without Cumber Stone affecting them. They enter with 4 power.
In fact, Warstorm only cares about the creature's power *when the trigger resolves*. This means that with the trigger on the stack, you could cast Aether Meltdown on one of them and Surge will then only cause that token to deal 1 damage since its power is now 1.
Cumber Stone
Warstorm Surge
Omnath, Locus of Rage
Your opponent is right in that Cumber Stone doesn't apply until they are already on the field (how would they apply any sooner anyway?). However, that is good enough for Warstorm Surge. There is never a time when those creatures are on the battlefield without Cumber Stone affecting them. They enter with 4 power.
In fact, Warstorm only cares about the creature's power *when the trigger resolves*. This means that with the trigger on the stack, you could cast Aether Meltdown on one of them and Surge will then only cause that token to deal 1 damage since its power is now 1.