What matters is the state of the board at the moment that the damage was dealt:
603.10. Normally, objects that exist immediately after an event are checked to see if the event matched any trigger conditions, and continuous effects that exist at that time are used to determine what the trigger conditions are and what the objects involved in the event look like. However, some triggered abilities are exceptions to this rule; the game “looks back in time” to determine if those abilities trigger, using the existence of those abilities and the appearance of objects immediately prior to the event. The list of exceptions is as follows:
None of the exceptions apply to things that trigger upon damage being dealt.
This means that when damage is dealt, the game looks at the state of the board to see if there are any triggered abilities that trigger. And as per rule 113.6 detailed in Glorfindel's answer, Atemsis is in the graveyard at that time and thus its ability does not function.