Sentinel of Ascension

A crossing between a large demonic hound and a drake who guards The Locked Altar. It has a bony appearance, with black fur growing over its body. Two large wings come forth from its back, however, the creature is unable to use them to fly. Between the sections that are covered with fur, particularly on it's back, large holes open revealing a stone-like pitch-black interior that gives off a faint orange glow. This part of its body is made of a bland of blackstone and obsidian.
It is very aggressive, striking out at any creature who enters the altar. When faced with multiple foes, it will let out a howl that calls forth a molten leviathan serpent to aid it. If the sentinel is defeated, the serpent will leave.

Traits

• Evasive: The sentinel of ascension has advantage on dexterity saving throws.
• Shielding Wings: Attacks that are directed from the sentinel's side will not harm the sentinel but instead its wings. Each wing can take 30 damage until it is too damaged to shield the sentinel. Any excessive damage after destroying a wing does not carry over to the sentinel unless the attack dealt at least half of the wing's max hp. The wings do not share any of the sentinel's resistances or immunities except its fire resistance. The wings are vulnerable to ice damage.
• Ignitable Core: The sentinel of ascension is able to draw in heat, igniting its core. Whenever it absorbs a source of extreme heat it gains a stack of burning core. When having at least one stack of burning core, any creature that attacks the sentinel with a melee attack will take 1d4 fire damage per stack of burning core over 2. When taking at least 25 fire damage in one round (before mitigation), the sentinel obtains a stack of burning core. The more stacks of burning core the brighter the core of the sentinel glows.
• Multiaction: The sentinel of ascension can make three actions on its turn.

Actions

• Claw: Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 14 (2d8 +5) slashing damage.
• Bite: Melee Weapon Attack: +7 to hit, reach 5 ft., one target. Hit: 17 (2d10 +6) piercing damage. If the target is prone an attack roll of 18 or higher counts as a critical hit for this action.
• Pounce: The sentinel of ascension leaps towards a creature within 40 ft. and jumps on it. The target creature must succeed on a DC 17 Dexterity saving throw or take 11 (2d10) bludgeoning damage and become restrained. On a successful save, the creature takes half damage but does not become restrained. A restrained creature can spend an action to attempt a DC 17 Strength check to escape the restrain.
• Channel Heat: The sentinel of ascension may absorb nearby sources of extreme heat within 20 ft., gaining one stack of burning core per head source. Doing so forces the sentinel to spend its bonus action on the start of its next turn to cast Flaming Beam, Scorching Field, Hail of Ember Bolts, or Phoenix Mimicry. After using Channel Heat, one of the sentinel's body parts starts glowing the same orange hue corresponding to the attack it will cast next round.
• Flaming Beam: The sentinel of ascension channels heat to its mouth, firing off a piercing beam of plasma in a straight line in front of it. The beam has a range of 120 ft. Any creature caught in the beam must succeed on a DC 16 Dexterity saving throw, or take 6 (1d12) fire damage for each stack of burning core. On a successful save a creature takes only half damage. By using Channel Heat, this ability can be used as a bonus action. Consumes all stacks of burning core.
• Scorching Field: The sentinel of ascension channels heat to its front legs and smashes into the ground with them, creating a 10 ft. radius circular explosion around itself. Creatures caught in the explosion take 5 (1d10) fire damage for each stack of burning core. By Channel Heat, this ability can be used as a bonus action. Consumes all stacks of charged core.
• Hail of Ember Bolts: The sentinel of ascension channels heat to the spikes along its back and tail, firing of a volley of flaming spikes in the air that will land around it in an 80 ft. radius donut-shaped area with a 10 ft. radius clear circle around the sentinel. Creatures caught in the downpour must succeed on a DC 17 Dexterity saving throw or take 5 (2d4) fire damage per stack of burning core. On a successful safe, a creature only takes half damage. By using Channel Heat, this ability can be used as a bonus action. Consumes all stacks of burning core.
• Phoenix Mimicry: The sentinel of ascension channels heat to along a vein-like pattern across its body, restoring damage done to it. The sentinel heals itself for 5 (1d10) hp per stack of burning core, being able to choose how to distribute the healing between its body and its wings. By using Channel Heat, this ability can be used as a bonus action. Consumes all stacks of burning core.
• Call Molten Levitan Serpent: As a bonus action, the sentinel of ascension lets out a loud howl that summons a nearby Molten Levitan Serpent. After using this ability it will take 1d6 +4 turns for the serpent to arrive.

Legendary Actions

• Disengage: The sentinel of ascension disengages and can move up to half its speed.
• Claw 
• Bite 
• Pounce
• Channel Heat

STR	DEX	CON	INT	WIS	CHA
20 (+5)	21 (+5)	18 (+4)	10 (+0)	16 (+3)	7 (-2)

Type: Feral Higher-demon
Armor Class: 17
Hit Points: 215
Speed: 45 ft.
Proficiency: +4
Saving Throws: STR +9, DEX +9
Skills: Acrobatics +9, Perception +7
Damage Resistance: Piercing, Slashing, Bludgeoning from non-magical sources; Fire
Condition Immunities: Charmed, Frightened
Senses: Sight 60 ft., Smell 120 ft.
Languages: -
Challenge: 12 (8,400)