Freaky Fast F*cker. As a bonus action, the deer can dash (3 charge, recharge: 6)

Ethereal Jaunt. As a bonus action, the deer can magically shift from the Material Plane to the Ethereal Plane, or vice versa.

Transform. When deer hits half health (100 hp), after shrieking, it changes form, losing all ongoing status effects, sprouting long spindly legs and having bony spikes erupt from its antlers and spine.  It then gains Spike Regrowth, Spider Climb, and Spike Throw features.

Spike Regrowth. The deer has twenty-four spikes. Used spikes regrow when the deer finishes a long rest.

Spider Climb. The deer can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Multiattack. The deer makes two attacks: with its bite , its blinding spittle or if under 100hp, its spikes.

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 15 (2d10+4) piercing damage.

Gore. Melee Weapon Attack: +7 to hit, reach 5 ft., one target.  Hit:  24  (3d12+5) the deer must move at least 10 ft in a straight line immediately before using this attack.

Spike Throw. Ranged Weapon Attack: +5 to hit, range 100/200 ft., one target. Hit: 9 (2d8 ) piercing damage + 13 (3d8) poison damage (Con save DC 14 to halve)

Shriek.  Upon being reduced to 100 hp, the deer lets loose an unearthly shriek.  All creatures within 100 ft must make a Wisdom saving throw (DC 14) or be frightened of the deer (Save ends).

Legendary Resistance x1