Amalgamation Slime

Spell Resistance. The Amalgamation Slime has advantage against saving throws against spells.

Undead Fortitude. When the Amalgamation Slime takes damage that would reduce its Hp to 0, it rolls a constitution saving throw against the DC equal to the amount of damage taken. If it succeeds, the Amalgamation Slime is instead reduced to 1 Hp.

Acid Splash. Whenever the Amalgamation Slime is hit, it secretes an acidic ooze out of its body that splashes those who are nearby. Anyone within 5ft of the Amalgamation Slime must make a DC 17 constitution saving throw taking 4d6(12) acid damage, and half on a successful save.

Spellcasting. The Amalgamation Slime is a 14th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). The Amalgamation Slime has following Wizard spells prepared:

Cantrips (at will): Acid Splash, Poison spray, Chill Touch, Infestation, Toll of the Dead 
1st level (4 slots): Dissonant Whispers, Grease, Ray of Sickness, Tasha's Hideous Laughter

2nd level (4 Slots): Darkness, Hold Person, Ray of Enfeeblement

3rd level (3 slots): Confusion, Vitriolic Sphere

4th level (2 slots): Cloudkill, Negative Energy Flood

5th level (1 slots): Circle of Death

Multiattack. The Amalgamation Slime can attack 3 times, or use its Engulf as a full action.

Pseudopod. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 18 (3d8 + 6) [Bludgeoning] damage and 12 (4d6) acid damage. On hit the creature can be grappled rather than take the bludgeoning damage (target will still take acid damage). To break the grapple the target can use its action to make an athletic, or acrobatics check versus DC 17. The target takes 12 (4d6) acid damage at the beginning of the Amalgamation Slimes turn until the grapple is broken. The Amalgamation Slime can grapple up to 2 targets at a time.

Greatsword. Melee Weapon Attack: +12 to hit, reach 5 ft., 1 target. Hit: 12 (2d6 + 6) slashing damage and 12 (4d6) acid damage.

Spear. Ranged and Melee Weapon Attack: +12 melee or +6 ranged to hit, range 20/60 ft., 1 target. Hit: 9 melee or 3 ranged (1d6 + 6 or 1d6 - 3) [Piercing] damage, and 12 (4d6) acid damage.

Engulf. If the Amalgamation slime has a creature grappled then it can use its action to submerge the target into its body, in which the target is considered suffocating and restrained. The target takes 12 (4d6) acid damage per round until the creature escapes. The target can use its action to make a DC 17 strength saving throw to escape the creatures grasp.

Frightful Presence. Each creature of the Amalgamation Slime's choice that is within 120 feet of it and is aware of it must succeed on a DC 17 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Amalgamation Slimes frightful presence effect for 24 hours.

Legendary Resistance 3/day. If the Amalgamation Slime fails a saving throw, it can instead choose to succeed using one of its legendary resistances.

The Amalgamation slime has 3 Legendary action points to use for the following abilities described below.

Move (Costs 1 Action). The Amalgamation slime can make its full movement at the end of a players turn.

Attack (Costs 1 Action). The Amalgamation can make a single Pseudopod attack versus a single target.

Skeletal Sentinel (Costs 2 Actions). The Amalgamation can use 2 of the skeletons stored in its body and summon them as minions. The Skeletal Sentinels move and act on the Amalgamation Slimes initiative order. See stats below.

Skeletal Sentinel

Hp: 35 AC: 12 Speed: 30 Feet

Str: 17 (+3) Dex: 14 (+2) Int: 1 (-5) Con: 16 (+3) Cha: 5 (-3)

Acid Splash. When the Skeletal Sentinel is attacked, it splashes the attacker with 6 (3d6) acid damage.

Actions

The Skeletal Sentinal can make 2 Melee Weapon attacks.

Shortsword, Melee weapon attack. +8 to hit, 6 (1d6+3) slashing damage and 6 (2d6) acid damage.