Arcane Bastion. If the Arcane Leech has no Spell Points and gains them, it receives 10 temporary hit points. 

Arcane Conduit. If the Arcane Leech expends a Spell Point, all arcane spells cast within 30 feet of it deal +2 damage and their Spell save DC is raised by 2 until the start of its next turn.

Arcane Camouflage. As long as the Arcane Leech has at least 1 Spell Point, other creatures have disadvantage on ability checks to spot it while it is using  Stealth to hide.

Multiattack. The Arcane Leech makes two leg slash attacks.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 10 (2d6 + 3) piercing damage. 

Leg Slash. Melee Weapon Attack: +7 to hit, reach 10 ft., 1 target. Hit: 7 (1d12 + 1) slashing damage.

Arcane Blast. Ranged Spell Attack: +4 to hit, range 30 ft, 1 target. Hit: 16 (4d6 + 4) force damage. Using this attack costs the Arcane Leech 3 Spell Points.

Astral Shift. The Arcane Leech must have at least 5 Spell Points to use this action; it expends all Spell Points and enters the Astral Plane for 1 minute. After this time, it is forced back into the Material Plane and it cannot choose to leave the Astral Plane before this time.

Arcane Well. If a creature within 60 feet of the Arcane Leech expends a spell slot to cast an arcane spell, the Arcane Leech gains 1 Spell Point. If the spell targets the Arcane Leech, it gains 1 additional Spell Point.

Arcane Drain. If the Arcane Leech hits a creature with arcane spell slots with a bite attack, it consumes one unexpended spell slot of the lowest level the creature has available. The Arcane Leech heals 5 hit points per spell level of the slot and gains 1 Spell Point.

Enter the description for how legendary actions work for your monster here.

Legendary Action Name (Costs # Actions). Enter the description for your legendary action.