It received a good thing due to damage I dealt. That's a buff.
The buff comes from passive skill that activates in certain conditions.
This occurred when I rolled a 6 on my Fire Breathing. Would Fire Frenzy have happened if I didn't do that, or would I still have to be dealing with an opponent with +2?
It would still occur even at 5. You just discovered an immunity and one of Cryptlord's skills.
For that matter, is everything about the Cryptlord made to counter me? Because it definitely seems like everything I try to throw at him is being countered.
Don't forget to put a tinfoil hat on your head before next turn, it'd block my telepathy that forces you to make a bad move...
It's a boss, not random crippled undead that desperately tries to chew your face off, you'd expect it to have some powerful resists, skills and immunities, but they still do have weaknesses - it's just generally harder to find one out cause most of the time a boss will try to obliterate you much harder and much more effective than random zombie. All enemies counter some traits of each player in some way. Only implings counter everything you'd throw at them. That's why you met them only once - enemy that counters everything you can do and deals a lot of damage is not fun and isn't even a fair challenge.
You have to make several bad moves to find out what are strong and weak sides of a boss. The stronger the boss - the harder it is to find out a weakness, but beating them is rewarding. Stuff like guaranteed level-ups (can be multiple depending on boss' level!) and unique, powerful items.