Barnabus, the Eternal Hound

Keen Hearing and Smell. The hound has advantage on Wisdom (Perception) checks that rely on hearing or smell.

Loyal to the Alpha. Barnabus only follows commands from the person he considers his current Master. Only the Master is able to ride atop Barnabus. All other riders, including allies, are shaken off. The Master can command Barnabus to follow the commands of allies to temporarily change Mastership, but Barnabus is only willing to do this for 1 minute, who afterwards will revert to obeying the original Master. 

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

Figurine Mode. When not summoned, Barnabus takes the form of a tiny 3-headed hound statuette. Summoning Barnabus requires the Master to use an action to throw the Figurine and speak the command words, "Come here boy!" Barnabus can be summoned for up to 3 hours or until Barnabus' Hitpoints are brought to 0. Once used the Figurine cannot be used again until Dawn of the next day. The Master can choose whether Barnabus is summoned as a Small, Medium or Large creature, but this has no effects on his stats. (Barnabus is only rideable if he is summoned as Large.)

Bonded for Life. Barnabus has bonded his soul with that of Amel Uldreiyn, and considers Amel his Master.

Chill Bite. Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: (2d8 + 7) piercing damage plus  (2d6) fire damage. On a hit the target cannot regain hit points until the start of Barnabus' next turn, leaving behind a spectral bite mark. If the target is undead it also has disadvantage on attack rolls against Barnabus or his Master until the start of Barnabus' next turn

Black Fire Breath (Recharge 5–6). The hound exhales fire in a 20-foot cone. Each creature in that area must make a DC 15 Dexterity saving throw, taking (6d10) fire damage on a failed save, or half as much damage on a successful one. Black Fire ignores fire resistance, and treats Fire Immunity as Fire Resistance.

Bound to the Master. When Barnabus' Master or the Master's allies is about to take damage from an attack, Barnabus can use his reaction to jump in front of a single target of the attack and take the damage for them, provided the target is within 10 ft. If Barnabus takes damage for the Master, the Master receives advantage on all attacks for their next turn.