Regeneration? How?

So, I can't figure out how to use regeneration in Forge.
I've got something with regeneration, say Nightscape Familiar.
[*]I cast damnation, and I don't get an opportunity to respond to it with the regeneration.
[*]Next time, I activate the regeneration in advance, since the regeneration effect lasts until end of turn; and then cast damnation, and it dies anyways.
What gives?
In case this is a bug: v1.5.37
I've got something with regeneration, say Nightscape Familiar.
[*]I cast damnation, and I don't get an opportunity to respond to it with the regeneration.
[*]Next time, I activate the regeneration in advance, since the regeneration effect lasts until end of turn; and then cast damnation, and it dies anyways.
What gives?
In case this is a bug: v1.5.37