100% physical damage immunity and 3 (so far) breath attacks that do a full party wipe when they land. Who designed these encounters? This is basically a dragon with 6 minions in the beginning of act 2. You can't even prepare damage immunities because he does fire, thunder, and acid damage (so far).
FOLLOW THE D&D RULES. When you try to make up rediculous creatures with epic burp attacks, you end up deviating from the careful balance that makes 5e great. This encounter is insane.