Beelzebub

Ravenous Gluttony. Beelzebub cannot take damage unless it is from its own self inflicted sources.

Ever Growing Hunger. Beelzebub can eat any dead creatures on the battlefield with an increasing maximum of 5 after each Regurgitation* starting at 10.

Insatiable Hunger Manifest. Upon reaching 0 Hp, change attack pattern.

Tantrum. If less than 10 creatures are extinguished after a round, Beelzebub throws a tantrum that affects every creature on the ground, dealing 10d10 bludgeoning.

Regurgitation. Every round it spawns an Additional Grave Beholder as its hunger grows. It sucks up every dead creature on the map and burps up the remains. Forcing each creature within 300 ft radius to DC 20 DEX save or take (Number of creatures eaten) xd6. It also takes self damage afterwards. 

Wail (Costs 3 Actions). Beelzebub lets out a withering wail. Any creature within 120 feet of Beelzebub that can hear the wail must succeed on a DC 19 Constitution saving throw or gain 1 level of exhaustion.