Spellcasting. Algernon is a 12th-level spellcaster. Algernon's spellcasting ability is Intelligence (spell save DC 16, +9 to hit with spell attacks). Algernon has the following spells prepared:

Cantrips (at will): mold earth, fire bolt, produce flame

1st level (5 slots): burning hands, catapult, earth tremor, shield

2nd level (4 slots): aganazzar's scorcher, earthbind, shatter

3rd level (3 slots): dispel magic, erupting earth, fireball, Melf's minute meteors

4th level (3 slots): conjure minor elementals, wall of fire

5th level (2 slots): wall of stone, transmute rock, immolation

6th level (1 slot): sunbeam

Sustained Concentration. Algernon may concentrate on a utility spell and a damage-dealing spell simultaneously. He rolls concentration checks separately on each.

Lava Cushion. If Algernon falls into the lava, he takes no fire damage and half falling damage. On subsequent turns, he may use his movement to move directly upward on a jet of lava.

Quarterstaff. Melee Weapon Attack: +7 to hit (+7 bludgeoning damage, 5 (1d8 + 1) bludgeoning damage if wielded with two hands.

Control Clone. Algernon's clone must succeed on a Wisdom saving throw (DC 16) or become charmed for one minute. At the beginning of each of its turns, Algernon may force his clone to move up to five feet and make a melee attack against one creature within range. Algernon's clone may make a Wisdom saving throw at the end of each turn to end the effect

Mold Earth. As a bonus action, Algernon can cast mold earth. If he uses this spell to create cover, it creates 3/4 cover.

Accelerated Casting. If Algernon did no damage with his action, he may cast a spell up to 1st level as a bonus action.

Absorb Spell. As a reaction, Algernon can attempt to absorb a spell he can see being cast within 150 feet. Algernon and the spellcaster make contested Arcana checks. If Algernon succeeds, the spell is cancelled and the caster must make a DC 16 INT save. On a failed save, the caster takes 3d8 psychic damage and is under the poisoned condition. On a successful save, the caster takes half damage and is not poisoned. In either case, the caster loses the spell slot. When Algernon successfully absorbs a spell, he may use his next action to cast the spell himself. At the end of each subsequent turn, the caster may make a DC 15 CON save to end the poisoned condition.

Algernon may take up to three legendary actions per round.

Legendary Reaction. Algernon may expend one legendary action to take a second reaction. If he uses Absorb Spell for his second reaction, he must expend two legendary actions.

Teleport. Algernon may expend one legendary action to teleport up to 60 feet, as long as his route passes through the elemental vortex.

Spellcasting. Algernon may cast a spell, expending one legendary action per spell level.

When Algernon is reduced to 100 hit points, he amplifies the elemental vortex to cast fire shield on himself. He does not need to maintain concentration on this spell. 

Tether Essence. After being reduced to 100 hit points, Algernon may cast tether essence on his clone as an action. There is no saving throw.