Alright. You want something that will genuinely threaten five level 16 characters under 2024/5.5e maths — not a pushover sack of hit points. For 5 × Level 16 PCs, a deadly encounter sits around CR 21–23 for a solo --- ☠️ Dread Sovereign of the Black Sepulchre Gargantuan Undead (Mythic), Neutral Evil CR 22 (Mythic CR 24 when empowered) Proficiency Bonus +7 --- Armour Class 26 (Necrotic Carapace) Hit Points 420 (30d20 + 120) Speed 40 ft., fly 40 ft. (hover) --- STR 26 (+8) DEX 14 (+2) CON 22 (+6) INT 18 (+4) WIS 20 (+5) CHA 23 (+6) --- Saving Throws Str +15, Con +13, Wis +12, Cha +13 Skills Perception +12, Intimidation +13 Damage Resistances Cold, Fire, Lightning Damage Immunities Necrotic, Poison Condition Immunities Charmed, Exhaustion, Frightened, Paralyzed, Poisoned Senses Truesight 120 ft., Passive Perception 22 Languages All it knew in life, telepathy 120 ft. --- Traits Legendary Resistance (3/Day) If it fails a saving throw, it can choose to succeed instead. Aura of the Grave (30 ft.) Creatures of its choice in the aura: Have disadvantage on death saving throws Regain only half hit points from healing Take 10 necrotic damage at the start of their turn Reaping Presence When a creature drops to 0 HP within 60 ft., the Sovereign gains 20 temporary HP. Undying Engine If reduced to 0 HP for the first time, it instead drops to 1 HP and unleashes Mythic Awakening (see below). --- Actions Multiattack Makes three attacks: two Grave Cleavers and one Soul Rend. --- Grave Cleaver Melee Weapon Attack: +15 to hit, reach 15 ft. Hit: 25 (3d10 + 8) slashing + 18 (4d8) necrotic. Target must succeed Con DC 21 or its max HP is reduced by the necrotic damage taken. --- Soul Rend (Recharge 5–6) 60 ft. line, 10 ft. wide. Creatures make DC 21 Con save. Failure: 72 (16d8) necrotic and restrained by soul chains until end of next turn. Success: Half damage, not restrained. --- Devour Essence Targets one restrained or unconscious creature within 30 ft. DC 21 Con save. Failure: 55 (10d10) necrotic and Sovereign heals same amount. If this reduces target to 0 HP, they must immediately make a DC 15 death save or die outright. --- Legendary Actions (3/round) Grave Step – Teleport 30 ft. Cleave (Costs 2) – Make one Grave Cleaver attack. Wave of Dread (Costs 3) – Each creature within 20 ft. must succeed DC 21 Wis save or be Frightened until end of next turn and take 22 (4d10) psychic. --- Mythic Awakening (Phase 2) When triggered: HP resets to 250. Aura expands to 60 ft. Gains Shadow Cataclysm (Recharge 6): Shadow Cataclysm All creatures within 60 ft. make DC 21 Con save. Failure: 90 (20d8) necrotic. Success: Half. Additionally, 4 Wraith Champions (use Wraith stat block with +40 HP) rise and act on its initiative. --- Why This Is Appropriate for 5 Level 16 PCs AC 22 pressures martials without making it unhittable. 420 + 250 mythic HP = 670 effective HP. Necrotic max HP reduction punishes reckless tanking. Healing suppression pressures support. Mythic phase prevents anticlimactic burst. Summoned adds break action economy in phase 2. Damage per round ~110–140 baseline, spikes higher on recharge