DEFENDER OF KHANDAQ: Black Adam can use Impervious, but if he would be dealt 4 or more damage, he reduces that damage by 3 instead of 2. His powers and abilities cannot be countered except for his Outwit.

vs

BURN THROUGH DEFENSES: When Atrocitus attacks a character that's using Impervious, this turn that character uses Invulnerability instead. When Atrocitus attacks a character that's using Invulnerability, this turn that character uses Toughness instead.

Would black adam still keep the -3 dmg reducer, or would ole' arti here ignore it? The part that confuses me is the "but" part. He can use Impervious, BUT ... what if hes using invul instead?

Yes, he would still get the -3. See this thread for the ruling (or at least one Rules Deputy's opinion on the subject).

I'd rule:

The -3 comes from the Defender of Khandaq SP - not from "Impervious". While the Imperv would change to Invulnerable, the -3 would still stick as Adam doesn't lose DoK.

Actually it's because from the "instead of -2" as Invulnerable also reduces -2. If a second Atrocitus attacked Black Adam the same turn, the Invuln would be come Toughness which would still only reduce -1.

Thanks, i'll bring it up there and close this one!