False Trophy.  While mounted on the wall, the demon elk appears to be a mundane hunting trophy. A creature that succeeds on a DC 17 Intelligence (Investigation) check while examining it can tell something is wrong. When a creature comes within 5 feet, the elk head animates, its eyes glowing red, and the full demon elk bursts forth from the wall.

Charge.  If the demon elk moves at least 20 feet straight toward a creature and then hits it with a gore attack on the same turn, the target takes an extra 14 (4d6) piercing damage. If the target is a creature, it must succeed on a DC 16 Strength saving throw or be knocked prone.

Frightful Presence.  Each creature of the demon elk’s choice within 30 feet that can see it must succeed on a DC 14 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Multiattack. The demon elk makes two attacks: one with its gore and one with its hooves.

Gore. Melee Weapon Attack Roll: +8, reach 10 ft. Hit: 18 (2d10 + 5) Piercing damage. 

Hooves. Melee Weapon Attack Roll: +8, reach 5 ft. Hit: 14 (2d8 + 5) Bludgeoning damage. 

Wall Burst (Recharge 5–6). The demon elk melts into a nearby wall and re-emerges up to 30 feet away, trampling through creatures in its path. Each creature in the line must make a DC 16 Dexterity saving throw, taking 27 (6d8) force damage on a failed save, or half as much damage on a success.

Wall Step.  As a bonus action, the demon elk can partially retreat into walls, pillars, or other vertical surfaces. While in this form, it has half cover and can’t be targeted by opportunity attacks. If reduced to 0 hit points while in a wall, it is banished back to the Abyss in a burst of flame and smoke.