Boss Encounter 20 - lvl 5 to 10

Glavenus Huge beast , unaligned
Armor Class 18 Natural Armor Hit Points (17d12 + 95) Speed 50 ft.
STR 25 (+7) DEX 10 (+0) CON 21 (+5) INT 16 (+3) WIS 12 (+1) CHA 10 (+0)
Saving Throws STR +12, CON +10, CHA +5 Skills Perception +6 Damage Resistances Fire Senses Passive Perception 16 Languages -- Challenge 13 (10,000 XP) Proficiency Bonus +5
Actions Multiattack. The glavenus makes one bite attacks and one tail slash attack, or it uses its sharpening and then its tail spin.
Bite. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 20 (3d6 + 5) piercing damage + (1d6) fire damage.
Tail Slash. Melee Weapon Attack: +12 to hit, reach 15 ft., one target. Hit: (3d10 + 5) slashing damage.
Sharpening. The glavenus sharpens its tail. The next two tail Slashes deal an extra (2d8) of fire damage.
Tail Spin (Recharge 5-6) After Sharpening, the glavenus launches itself forward, using its tail to slash at all foes in 15 foot radius around it. Each creature in that area must then make a DC 16 DEX saving throw, taking (6d6) slashing and (6d6) fire damage on a failed save, and half as much on a successful one.