Blood of the Wolf. When the abyss watcher reaches 0 HP, all nearby undead legionnaires die. After one round, the abyss watcher resurrects, fully restoring its HP. It gains immunity to fire and all its attacks unleash a fiery aura, modifying the effect of each attack.

Multiattack. The abyss watcher makes one dagger and two greatsword attacks.

Farron Greatsword. Melee Weapon Attack: +7 to hit, reach 5 ft., 1 target. Hit: 19 (4d6+5) slashing damage, plus 7 (2d6) fire damage in phase 2.

Parrying Dagger. Melee Weapon Attack: +9 to hit, reach 5 ft., 1 target. Hit: 12 (3d4+5) slashing damage.

Greatsword Lunge (Recharge 4-6). The watcher lunges in a straight line at a target of its choice within 60 ft., dragging its sword across the floor while doing so. All creatures in this line must succeed at a DC 16 Dexterity saving throw or take 33 (8d6+5) slashing damage plus 7 (2d6) fire damage, or half as much on a successful save. The fire damage is only applied during phase 2.

Leap. As a bonus action, the watcher may make a 30 ft. leap in any direction. This movement may provoke attacks of opportunity.

Rally. As a bonus action, one other creature of the watcher's choice that it can see gains 12 temporary HP.

Parry. When being targeted by a melee attack, the abyss watcher may use its reaction to increase its AC by 5, potentially causing the attack to miss. 

Riposte. When an an attack misses the abyss watcher, it may use its reaction to make an attack against the creature that attacked it.

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

Retreat (Costs 1 Action). The watcher leaps up to 30 feet away. This movement may not provoke attacks of opportunity. 

Rush attack (Costs 1 Action). The watcher makes a greatsword attack, albeit with a reach of 15 ft.

Legion (Costs 2 Actions). The watcher invokes the pact of the wolf, calling for assistance from its fallen brethren. Roll a d2 and apply one of the following effects depending on the result of the roll:
1: An undead legionnaire rises from its slumber and assists the abyss watcher in combat.
2: An undead legionnaire rises, but is unable to distinguish friend from foe and is hostile to all creatures.

This legendary action may only be taken during phase 1, and only 2 legionnaires may be active at a time as a result of this action.

Slam (Costs 2 Actions). The watcher slams its blade into the ground, igniting the area around it. All creatures within a 30 ft. radius must make a DC 16 Constitution saving throw. On a failed save, they take 28 (8d6) fire damage and fall prone, taking half as much damage and staying upright on a successful save. This action can only be taken during phase 2.