Boss Encounter 15 - lvl 5 to 10

Giant Diving Bell Spider Large beast , unaligned
Armor Class 17 Natural Armor Hit Points (13d12 + 52) Speed 30 ft., swim 30 ft., climb 30 ft.
STR 18 (+4) DEX 14 (+2) CON 18 (+4) INT 15 (+2) WIS 10 (+0) CHA 8 (-1)
Saving Throws DEX +6, CON +8 Skills Stealth +7 Damage Resistances Fire Condition Immunities Poisoned Senses Blindsight 15 ft, Darkvision 60 ft, Passive Perception 10 Languages Common Challenge 12 (8,400 XP) Proficiency Bonus +4
Anphibious. This Spider can breathe both water and air.
Spider Climb. The spider can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Web Sense. While in contact with the web, the spider knows the exact location of any other creature in contact with the same web.
Web Walker. The spider ignores movement restrictions caused by webbing.
Actions Multiattack. The elemental makes two pierce attacks and one bite attack or one Whelming Web Bubble attack.
Pierce. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: (2d8 + 4) bludgeoning damage.
Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one creature. Hit: (2d8 + 3) piercing damage, and the target must make a DC 11 CON saving throw, taking (2d8) poison damage on a failed save, or half as much damage on a successful one. If the poison damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.
Whelming Web Bubble. Ranged Weapon Attack: +5 to hit, range 30/60 ft., one creature. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 12 STR check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 10; hp 5; vulnerability to Fire; immunity to bludgeoning, poison, and psychic damage). In addition, any creature caught in the web must make a DC 16 CON Saving Throw or become unconscious. Every time a character succeeds they gain a point of exhaustion.