Ok so let's say my opponent has a Festering Mummy (It's a 1/1 and when it dies put a -1/-1/ counter on target creature) and I cast Splendid Agony (Distribute two -1/-1 counters among one or two target creatures) and I put a -1/-1 on the Festering Mummy, is it considered to have died and does its death effect still occur?
Creatures that have 0 or less toughness will "Die" according to state-based actions.
This is covered by the following two rules:
704.5f: If a creature has toughness 0 or less, it’s put into its owner’s graveyard. Regeneration can’t replace this event.
700.4: The term dies means “is put into a graveyard from the battlefield.”
So for your specific example what will happen is:
- You cast Splendid Agony targeting your opponent's Festering Mummy.
- It resolves placing one -1/-1 counter on Festering Mummy.
- State-based actions see that Festering Mummy has 0 toughness, and moves it to the graveyard (i.e. Festering Mummy dies).
- Festering Mummy's death ability triggers and goes on the stack.