Crystalline Sentinel (WBtW)

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

Magic Resistance. The crystalline sentinel has advantage on saving throws against spells and other magical effects.

Magic Weapons. The crystalline sentinel's weapon attacks are magical.

Unusual Nature. The crystalline Lookout does not require air, food, drink, or sleep.

Multiattack. The crystalline sentinel makes two Slam attacks AND one Gemstone Bolt attack.

Slam. Melee Weapon Attack: +11 to hit, Reach 5 ft., one target. Hit: 10 (2d6+3) bludgeoning damage.

Gemstone Bolt. Ranged Weapon Attack: +8 to hit, Reach 120 ft., one target. Hit: 17 (3d4) magical damage.

Roll a d6 when attacking to determine the type of bolt fired: 

1-2: Diamond Dart (piercing damage)
This attack is guaranteed to hit and ignores any resistance or immunity the target has to piercing damage. 

3-4: Sapphire Spike (slashing damage)
The target must make a DEX Saving Throw (DC 14) or take an additional 2d6 slashing damage from the jagged sapphire.

5-6: Ruby Ray (fire damage)
The target must make a DEX Saving Throw (DC 14) or be set on fire, taking 2d6 fire damage at the start of each of its turns until it or another creature use an action to put out the flames. 

Reflective Light (Recharge 5-6). The crystalline sentinel emits a bright light in a 15-foot radius. Any creature within the radius must make a DC 14 CON saving throw or become [condition]blinded[/condition] until the end of its next turn.