Large Construct, Unaligned

Armor Class: 19 (Natural Armor)

Hit Points: 123 (13d10+52)

Speed: 30 ft. 

STR 18/+4

DEX 10/+0

CON 18/+4

INT 8/-1

WIS 14/+2

CHA 6/-2

Saving Throws: Constitution +7
Damage Immunities: Poison, Psychic

Condition Immunities: Charmed, Exhaustion, Frightened, Paralyzed, Petrified, Poisoned

Senses: Darkvision 120 ft., Passive Perception 12

Languages: Understands the languages of its creator but can’t speak

Challenge: 8

Proficiency Bonus: +3

Arcane Battery. Designed as a way for wizards to store their spells and magical energies for later use. The Battery Golem can store and utilize spells that were cast at it or near it (Through the Spell Store Reaction). Battery golem can cast spells stored inside it without requiring any components and with the spellcasting DC of 16. 

(Battery Golem is likely to have some spells stored inside it, so here is a basic template for spells that a combat focused Battery Golem could have)

• Greater Invisibility (4th level)
• Haste (3rd level)
• Lightning Bolt (3rd level)
• Miniature Meteors (3rd level)
• Mirror Image (2nd level)
• Slow (3rd level) 
• Web (2nd level) 

When a Battery Golem uses a stored spell, that spell is expended and cannot be used anymore. 

When within 60 feet of its creator, the creator can cast and expend spells stored inside the Battery Golem using their own spellcasting DC, if the spell inside the golem is one of their known spells. The creator decides if they want the spell to originate from them or from the battery golem, and who of the two has to maintain concentration on the spell if required. 

The Battery Golem can without issues hold spells with the cumulative spell slot level of 20 at a single time. If the Battery Golem were to exceed this capacity, they take 5 force damage for each spell slot level above its capacity at the start of each of its turns.  

Residual Arcane Force. Each time Battery Golem casts a spell, its next successful melee attack made until the end of the next turn deals extra 1d8 force damage for each spell slot level of the spell. 

Actions

• Slam. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 13 (2d12+5) bludgeoning damage.
• Magnetic Grab. (Recharge 5-6) Up to 3 creatures of the Golem’s choice within 60 feet of it must make a DC 16 Strength saving throw, being pulled 45 feet closer to the Golem, knocked prone, and taking 2d12 bludgeoning damage on a failure.  

Reactions (2x per turn)

• Spell Store. When targeted by a spell cast with a 5th level spell slot or lower, or if such a spell is cast within 15 feet of the Battery Golem. The Battery Golem must use its reaction to try and store the spell in itself. If the spellcaster is unwilling, they must succeed on a DC 16 Intelligence saving throw, otherwise their spell has no effect and the Battery Golem stores it in itself. Spells are stored inside the battery golem with the same spell slot level as they were cast.
  This feature doesn’t require a reaction if used against spells cast by the Battery Golem’s creator