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

Two Heads are better. The Underhound hound has two heads one with the Alignment of Lawful Good, the other Lawful Evil. Roll a D20 on an even number the LG head Takes control and gains advantage on the next attack and replace the secondary damage with radiant damage , if the number is Odd the LE head takes control; the Hound rolls with disadvantage but gains the effects of reckless attack and replaces the secondary damage with necrotic damage. Upon rolling a 1 both heads fight for control and the Hound skips its current tturn. If the roll is a NATURAL 20 both heads work in unison and the attack gains an extra +2 to its Attack and Damage rolls and gains the effect of both secondary damage types (Radiant and Necrotic)

Multi Attack. The Underhound makes two bite attacks

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:  1d4+2 piercing damage plus 2d4 Radiant or 2d4 Necrotic damage depending which head has control ,Or 1d4 Fire, 1d4 Radiant, and 1d4 Necrotic if both heads are in control.

Sacred/Necro Flame Breath (Recharge 5–6). The Underhound uses one of its heads to exhale fire in a 10-foot cone. Each creature in that area must make a DC 12 Dexterity saving throw, taking 2d6 fire damage an 2d6 Radiant or Necrotic (depending which head has control 2d6 Fire, 1d6 Radiant, and 1d6 Necrotic if both heads are in control) damage on a failed save, or half as much damage on a successful one.