Okay, here's what we've got so far:
Compared to a regular Nandie, the head Nandies of a colony in
Halls of Tizun Thane had triple the Hit Dice (3D8 vs 1D8) and did more damage (1d4/1d4/1d6 claw/claw/bite vs 1d2/1d2/1d3).
This version has 250% hit points rather than 300% like the original. (5d8 vs 2d8).
Maybe bump the Constitution up to give it +1 per die? That'd be pretty triple the base Nandie-Ape's 9 hit points:
Hit Points 27 (5d8+5)
CON 13 (+1)
For the attacks, I'm thinking the "to hit" might benefit from adding its Proficiency Bonus like the 5E
Ape (which seems a good model for the Boss and has a +3 Str bonus and +5 to hit).
Melee attack total average damage is up by 2 points (1d4+1d6+4 vs 1d4+1d6+2) or 10 vs 8, which is a 25% increase.
The original monster has 8.5 vs 5 (2d4+1d6 vs 2d2+1d3) or a 70% increase.
That suggest we need to give the damage a boost!
If we want to give them 170% or so of the regular Nandie-Ape's damage, that suggests the boss needs a total average damage of 13.5 or so. We could do that by giving the boss two claw attacks when multiattacking:
Multiattack. The boss nandie-ape makes three attacks: one with its bite and two with its claws.
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d4 + 2) piercing damage.
Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) slashing damage.
Rock. Ranged Weapon Attack: +4 to hit, range 25/50 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.
That seems just a tad too high in melee at 81.25% above the regular Nandie, being 14.5 vs 8 (2d4+1d6+6 vs 1d4+1d6+2).
However, I'd rather keep the two attacks and give it bigger and/or more dice:
Multiattack. The boss nandie-ape makes two attacks: one with its bite and one with its claws.
Bite. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (1d6 + 2) piercing damage.
Claws. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 5 (2d4 + 2) slashing damage.
Rock. Ranged Weapon Attack: +4 to hit, range 25/50 ft., one target. Hit: 5 (1d6 + 2) bludgeoning damage.
That's 56.25% above the regular Nandie, at 12.5 vs 8 (2d4+1d6+4 vs 1d4+1d6+2).
It's lower than a 70% would give, but the Nandie boss has more HP than the 5E Ape (27 vs 19), a point better AC and Advantage on attacks when using
Call to Arms.
Hmm… that's starting to look like a
Challenge 1 monster to me. Those numbers aren't far off a
Dire Wolf, whose Pack Tactics give it a similar Advantage-on-Attacks ability.
If we do go for the next rung up the Challenge ladder, I suggest giving them DEX 17 (+3) and increasing the attacks to "+5 to hit" like the Ape and Dire Wolf.
What do you think?
