Valkyrie Sentinel

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

Magic Resistance. The Valkyrie Sentinel has advantage on saving throws against spells and other magical effects.

Arcane Weapons. The Valkyries Sentinel's weapon attacks are magical.  When the Valkyrie Sentinel hits with any weapon, the weapons deals an additional 18 (4d8) force damage (included in the attack). 

Multiattack. The Valkyrie Sentinel makes two longsword attacks.

Longword. Melee Weapon Attack: +1o to hit, reach 5 ft., one target. Hit: 10 (1d8 + 6) slashing damage plus 18 (4d8) force damage.

Arcane Burst (Recharge 6). The Valkyrie Sentinel strikes the ground, creating a burst of arcane energy that ripples outward from it.  All creatures within 20ft of it must succeed on a DC 16 Constitution saving throw or take 21 (6d6) thunder damage, as well as 21 (6d6) force damage and be knocked prone. A creature that succeeds on its saving throw takes half damage and isn't knocked prone.

The Valkyrie Sentinel can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn.  The Valkyrie Sentinel regains spent legendary actions at the start of its turn.

Move. The Valkyrie Sentinel moves up to it's speed without provoking opportunity attacks.

Longsword (Cost 2 Actions). The Valkyrie Sentinel makes one longsword attack.