Cool, I look forward to hearing about it
So my players fought this thing last night. They are a large group of seven 13th-level characters, consisting of:
- a human barbarian
- a half elf diviner wizard/bard
- a human druid
- a gnome evoker wizard
- a dwarf life cleric
- a tiefling monk
- a human ranger (unearthed arcana variant)
To account for this, I gave the the retriever Legendary Resistance and three Legendary Actions with its eye rays. I modified its stats slightly, using most of yours with a bit of the stats from Planar Bestiary. I didn't bother to calculate its correct CR and just bumped it up to CR 18 instead. Here are its stats:
Retriever
Huge construct (demon), chaotic evil
Armor Class 20 (natural armor)
Hit Points 207 (18d12 + 90)
Speed 50 ft., climb 50 ft., swim 30 ft.
STR 24 (+7) DEX 14 (+2) CON 20 (+5) INT 3 (-4) WIS 11 (+0) CHA 4 (-3)
Skills Perception +5
Damage Resistances cold, fire, lightning Damage
Immunities poison, psychic; bludgeoning, piercing, and slashing from nonmagical weapons that aren't adamantine
Condition Immunities charmed, exhaustion, frightened, paralyzed, petrified, poisoned
Senses darkvision 120 ft., passive Perception 15
Languages Understands Abyssal but can't speak
Challenge 18 (18,000 XP)
Immutable Form. The retriever is immune to any spell or effect that would alter its form.
Find Target. When ordered to find an item or a creature, the retriever does so unerringly, as though guide by a locate creature or locate object spell, with no limit on the range. The being giving the order must have seen (or have an item belonging to) the creature to be found, or have touched the object to be located.
Legendary Resistance (3/Day). If the retriever fails a saving throw, it can choose to succeed instead.
Magic Weapons. The retriever’s weapon attacks are magical.
Indestructible. The retriever regenerates 5 hp at the start of its turn. While the retriever is at 0 hp, keep track of any damage dealt to it, including leftover damage from the attack that reduced it to 0 hp. If this damage total reaches 50 hp before the retriever can regenerate, then it is destroyed and can no longer regenerate.
Spider Climb. The retriever can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.
Innate Spellcasting. The retriever can innately cast the following spells, requiring no material components: 2/day each:
plane shift
ACTIONS
Multiattack. The retriever makes four claw attacks.
Claw. Melee Weapon Attack: +13 to hit, reach 10 ft., one target. Hit: 20 (3d8 + 7) slashing damage.
LEGENDARY ACTIONS
A retriever has three legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The retriever regains spent legendary actions at the start of its turn.
Eye Rays. The retriever’s eyes can produce one of four different magical rays, chosen from the options below. Each ray has a range of 100 feet and targets a single creature. Once an eye ray is used, the retriever cannot use the same eye ray attack until the end of its next turn.
Fire Ray. The targeted creature must make a DC 18 Dexterity saving throw, taking 42 (12d6) fire damage on a failed save, or half as much damage on a successful one.
Cold Ray. The targeted creature must make a DC 18 Dexterity saving throw, taking 42 (12d6) cold damage on a failed save, or half as much damage on a successful one.
Lightning Ray. The targeted creature must make a DC 18 Dexterity saving throw, taking 42 (12d6) lightning damage on a failed save, or half as much damage on a successful one.
Petrification Ray. The targeted creature must make a DC 18 Constitution saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.
Combat was short and sweet, the retriever lasted three rounds. Since this is an experiment, I simply had the retriever drop down on the party and attack. Half of them have the Alert feat or weapons of warning, so no surprise on those characters. My initiatives for the retriever were low, so the unsurprised characters hammered it hard (the barbarian, ranger, wizard, and monk) first. Still, those eye ray legendary actions got two out of three times in the opening round - the PC targeted with the petrification ray made his Con save, but the cold ray and lightning ray hit their targets (who failed their saves) for some heavy damage. During the fight, the diviner/bard cast Otto's Irresistable Dance to no effect. The retriever's claw attacks were vicious, it hit with all four attacks every time for a damage total of 80+ hp.
As I mentioned, the fight was quick. Seven attackers on one will always ensure a short battle, but I wasn't disappointed with the final results. I didn't want a long, drawn-out slugfest and this thing brought the pain. Although none of the PCs were dropped, they were all left pretty beat up afterwards. Not bad for one against seven powerful, very capable, high level characters. What surprised me is it didn't need to use Legendary Resistance once. The spellcasters opted for damage dealing spells and when a save was called for I ended up making the save with a dice roll. Hah go figure.
Now that I have tested it I feel pretty good about going back for round 2, which will definitely happen due to its Indestructible trait (the party did not think to check it afterwards). I'll use a different approach, obviously, with different tactics and/or environment.