Mysterious Stranger

You've gained the attention of a Mysterious Stranger, a guardian angel who aids you in combat:

• When rolling for Initiative, if you roll a 19 or a 20 on the die, the Mysterious Stranger will appear to aid you in combat. The Mysterious Stranger rolls his own initiative and is controlled by the DM.
• The Stranger only helps as a combatant, never healing or providing any other kind of aid apart from attacking your foes. The Stranger will always side with you, no matter, where combat goes. Your enemies are his; your allies his. If either of these roles change, the Stranger adjusts accordingly.
• The Stranger will disappear just as mysteriously as he appeared either when the combat encounter ends, he drops to 0 hit points or you fall Unconscious.
• Two Strangers can not appear in one encounter. If multiple players roll to summon him he simply helps all players involved in the successful summoning roll.
• The Stranger appears in a long grey cloak and hood. While his features are hard to make out, he appears to be some sort of adult male humanoid.

Mysterious Stranger

Medium humanoid, neutral

Armor Class 15
Hit Points half the hit point maximum of his summoner (you) 
Speed 30 ft.

Senses passive Perception 13

Languages all languages known to his summoner

Challenge -- (0 XP)

Mysterious. The Mysterious Stranger is just that; mysterious. Nobody notices him when he appears or disappears, always managing to be missed in the blink of an eye. When The Stranger drops to 0 hit points or lower he is not killed per se, merely disappearing from the fight just as unnoticeably as he appeared. This has no effect on the chances of him appearing again.

Scaled Assistance. The Mysterious Stranger is only as powerful as his summoner. His attack scales with the level of the player who summoned him (added to attacks.)

Improved Criticals. The Mysterious Stranger is a tremendous shot, and when he hits he hits effectively. The stranger hits critically on a 19 or 20 and critical hits deal triple dice damage as opposed to the normal twice.

Actions

Mysterious Bolt. The Stranger fires a bolt from his Mysterious Wand. He makes a spell attack to hit at a 60 ft. range and deals 13 (3d8) + Half the summoner's level (rounded down) force damage. His spell attack modifier is equal to the summoner's level.

Reactions

Finishing Move. If an enemy creature within 60 ft. of the Stranger is reduced to 1 HP by an attack, the Stranger uses his reaction to fire a Mysterious Bolt at the creature, instantly hitting and killing the target, regardless of immunities or resistances. Unless said creature has been specified by the summoner to be spared, the Stranger will always take this reaction at first opportunity.