Magic Resistance. The Xag-ya has advantage on saving throws against spells and other magical effects. Unless the spell deals cold or necrotic damage.

Death Burst. When the Xag-ya dies, it explodes in a burst of positive energy. Each creature within 10 feet of it must make a DC 14 Dexterity saving throw, taking 17 (4d6 + 3) radiant damage on a failed save, or half as much damage on a successful one. 

Mantle of Life.  An aura of positive energy flows from the Xag-ya in a 30-foot radius. Until the effect ends, this aura moves with the Xag-ya, centered on it. While in the aura, each non-hostile creature in the aura (including the Xag-ya) deals an extra 1d4 radiant damage when it hits with a weapon attack.  Additionally,  non-hostile creatures that start their turn in the aura, get 7 (1d6 +3) temporary hit points. 
Duration: Concentration, up to 1 minute. Uses an action to activate.

Multiattack. The Xag-ya makes two Tendril attacks.

Positive Energy Tendril. Melee Weapon Attack: +6 to hit, reach 10 ft., one target. Hit: 10 (2d6 +3) radiant damage.  A xag-ya’s incorporeal touch damages undead creatures, but heals living creatures for 10 (2d6 +3). A xag-ya can control its positive energy enough to avoid healing living foes, instead dealing 7 (1d6 +3)  radiant damage. A creature can only be healed by the tendril once per turn.

Positive Energy Bolt (Recharge 5-6). The xag-ya fires a bolt of positive energy at one target it can see within 30 feet of it. The target must succeed on a DC 14 Dexterity saving throw or take 7 (1d6 + 3) radiant damage. If the target is a creature or a flammable object, it ignites and burns with radiant fire for 1 minute. Metallic objects hit by the bolt glow white hot for 1 minute as if under the effects of a heat metal spell. The fire can be extinguished with magic that deals cold damage or similar effects or by casting dispel magic or similar spell upon it. Until the fire is extinguished or goes out, the target takes 2 (1d4) radiant damage at the start of each of its turns.