Hi there, creating a custom boss to a high fantasy party and need your input to make sure it’s not too much or just right.

It is a high fantasy campaign mixing science fiction elements. The party will get locked into a bunker and have to fight a sentry and its flying targeting friend. 

The party is level 5 and they are about 4-5 players, they have two NPC’s with them most likely for this fight - a CR 5 vampire, and a CR 5 troll (hah don’t ask, long story).

So I balanced this to be about a CR 9 or 10, it is probably too hard for the individual PC’s but since the NPC’s are there, I figured I needed to boost the encounter CR to atleast a 9 or 10.

The sentry:

Str 10, dex 20, con, 14, int, 14, wis 9, cha 10

AC 12

HP 205

Move speed: Nil

Damage immunities: all attacks except spells but loses immunity when targeter is damaged until the targeter’s next turn.

Condition immunities: charmed, frightened, poisoned, paralyzed, petrified, exhaustion.

Attacks (+7 attack bonus for all)

2d10 +5 Raybeam

Petrifying gaze (DC 14)

Color Spray

Magic Missile (can attack up to 5 targets with one magic missile)

Sleep

Abilities: Mirror image (recharge 4-5)

-

The Targeter:

Str 10, dex 14, con 10, int 12, wis 10, cha 8

AC 16 (20 with shield)

Hit points: 101

Move speed: Fly 30 ft

Immune to all magic

Spells: Shield (recharge 4-5)

Same condition immunities as sentry

When the targeter locks on to a subject (or subjects) the sentry on its next turn can attack.

When the targeter is damaged, the sentry for one round can be hit by all attacks (until the targeter’s next turn)

If the targeter is destroyed, the Sentry can no longer target and can only cast magic missile randomly at all targets in the area until dead.

Sentry core:

When the sentry is destroyed, it releases its core.

Str: 8, dex: 30, con: 1, int: 10, wis: 10, cha: 10.

HP: 1

AC: 22 (26 with shield)

Move speed: fly 50 ft

Same condition immunities as sentry and targeter

Uncanny dodge (successful reflex save negates damage)

Has anti magic field 20 ft around it

Spell: shield (3x/day)

Special abilities: Disintegration beam - all party members and NPC’s in the area will have disintegration cast on them after 3 rounds of announcing the attack.

Is it too hard/insane for a level 5 party with two CR 5 NPC's? What CR would you give this encounter? What can I tweak or is it awesome?

I pretty new so unlikely to be able to help much. But just reading it is the idea that they attack the targeted first then the sentry?

My only thought coming at it from a PC trying to attack it is if the targeter is immune to all magic damage, has A.C. 20, and flies you are putting a lot on only team members with no magic range attacks. So probably depends on party makeup? Else you could have a lot of people unable to do anything on their turn...

Could be completely wrong mind. Just browsing from my phone :D

I love the concept and the flow is super-creative, but I do have to agree with GaspodeWD about putting a lot on the fraction of the team with non-magic range attacks, and would practically exclude any team members who's focus is non-magic melee combat (exempli gratia, the troll) if the targeter fails to be damaged in a given turn.

I would probably reverse the damage immunities between the targeter and the sentinel, id est, make the targeter immune (or at least resistant) to physical damage and the sentinel immune to all magical damage (and possibly resistant to physical slashing/piercing/bludgeoning damage as well if you feel like its hit points are going to be eaten up too fast) unless the targeter takes damage (or, fails a CON save, as though concentrating on a spell). That way, all characters can contribute even if the targeter escapes damage for a given round.

The sentinel core, especially with an armor class of 26 and three uses of the shield spell will be practically impossible to hit for a 5th-level party (exempli gratia, an ancient gold dragon only has an armor class of 22). I'd be more inclined to give the sentinel core a few more hit points (enough to survive two normal hits, but only one critical hit, as an idea) and drop the AC considerably. After all, it is more fun to hit a monster and have it stand back up than it is to miss entirely.

Anyway, that's where I'm at. I hope that it helps!

Hmmmm, after reading what you guys are writing, I was thinking about this, the party has a barbarian with javelins and a ranger with a sling (who does a ****ton of damage believe it or not), The magic users can target the sentry with all their spells, but perhaps to allow more party members to hit the targeter I could take away its fly ability... it's just less cool. And the issue would be it would die in moments because of the two level 5 NPC's if they can swing at it, so I was trying to prevent that. If  Ido this I'll double its hit points.

In regards to the final 'core' I want it to be super stressful for the party. Hearing a countdown where they'll all get hit with disintegrate and it just needs to be tapped by 1 damage sounds very exciting :P. I included the antimagic field and uncanny dodge to exclude cheesing it with a splash spell or magic missile, so they'd have to figure out how to just *hit it once. I think maybe lower its AC to 17 and giving it shield might make it better/ more fair.

"The magic users can target the sentry with all their spells"

But with magic immunity they can't damage it. 

I agree that keeping it flying is better, you just need to change the immunities for the two parts so that people can at least do something each turn. Maybe switch from immune to resistant? 

No no no, the magic users at all times can attack the sentry, but the physical damage users can't damage it unless the targeter gets damaged.

Ah I'd read it as the sentry was also immune to magic. 

So casters will attack the sentry and martial will attack the targeter...