Obelisk Spider

If Obelisk Spider deals combat damage to a creature but that creature isn’t on the battlefield as Obelisk Spider’s first triggered ability resolves, most likely because Obelisk Spider killed that creature already, that ability won’t put a -1/-1 counter on it.

Because damage remains marked on a creature until it’s removed as the turn ends, the counter Obelisk Spider’s first triggered ability puts on a creature may cause previously marked damage to become lethal.

If a creature with wither or infect deals damage to a creature, the controller of the creature with wither or infect puts that many -1/-1 counters on the second creature.