Betrayer

Pack Tactics. The Betrayer has advantage on an attack roll against a creature if at least one of the Betrayer's allies is within 5 feet of the creature and the ally isn't incapacitated.

Bio light sensitivity: Whenever the Betrayer starts its turn within artificial light, either from Lumidogs or Illushrooms, It is blinded until it leaves the area causing the artificial light.

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

Reactive. The Betrayer can take one reaction on every turn in a combat.

Blood soaked teeth: The Betrayers attacks ignore resistance to damage and counts immunity as resistance.  

Pack prowlers: For each additional Betrayer within 30 feet of the Betrayer, it gets +1 to AC, +1 to its attack rolls and +1 to damage rolls.

Serrated teeth: For each pair of betrayers within 30 feet of the Betrayer, its attacks deal an additional 2d4 damage on its attacks. 

Serrated Bite. Melee Weapon Attack: +13 to hit, reach 5 ft., one target. Hit: 17 (3d4+8) piercing damage, plus 24 (4d10) necrotic damage is the target is already wounded.

Bolster the pack (Recharge 6). The betrayer unleashes a loud ear curdling howl. Each betrayer within 30 feet of it gains resistance to non magical damage for 1 minute. If the Betrayer is at half or less maximum hp, Each Betrayer gains an additional 25 feet of movement and may take the Dash or Attack action as a reaction.

  

Internal hatred: Whenever a Betrayer takes damage, it may make an attack of opportunity against its attacker if within range.