Blood Giant

Legendary Resistance (3/Day). If the giant fails a saving throw, it can choose to succeed instead.

Ageless Nature. The giant does not age.

Immutable Form. The giant is immune to any spell or effect that would alter its form.

Magic Weapons. The giant's weapon attacks are magical.

Innate Spellcasting. The giant's innate spellcasting ability is Charisma (spell save DC 18, +10 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: command, earthbind, dispel magic

3/day each: counterspell, compulsion, geas

Multiattack. The giant makes three greatsword attacks.

Greatsword. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 24 (6d6 + 6) slashing damage.

Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 15 (3d6 + 6) piercing damage. If the target is a creature, it is grappled (escape DC 18). Until this grapple ends, the target is restrained, and the giant can't bite another target.

Swallow. The giant makes one bite attack against a Medium or smaller creature it is grappling. If the attack hits, that creature takes the bite's damage and is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the giant, and it takes 21 (6d6) acid damage at the start of each of the giant's turns.

If the giant takes 15 damage or more on a single turn from a creature inside it, the giant must succeed on a DC 15 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the giant. If the giant dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Manifest Greatsword (Recharges after a short or long rest).  The giant fashions a greatsword out of its own blood through innate magical means and equips it, as long as it is not already wielding a weapon in its hands.