Help me buff this oni!

[Inevitability]

For a campaign I'm running, I am planning on having the players encounter a half-black dragon oni (currently masquerading as an ally of the PC's) at some point. The problem; they probably won't encounter it until level 13-15, a point where a single oni is far beyond interesting anymore.

So my question is; how to buff up this oni? All help is welcome!

[jazzymantis]

Magic Items like mantle of spell resistance, +1 ac ring, wondrous figurines for minions to fight? Magic glaive, something like +1-2 or make it a frostbrand/flametounge glaive

What is the CR for a half dragon Oni? Maybe buff the dragon abilities? Boost HP? Change multiattack to 3?

Up the level of spellcasting? possibly move some of the 1/day spells to at will or increase to 3-4/day. Add cantrips like ray of frost or chill touch,
Higher level/more spells like Armor of agythys, hex, mirror image, crown of madness, misty step, see invisibility, suggestion could be 1-3/day. Blink could also be fun, and would really mess with players.
Other spells could be vampiric touch, confusion, greater invisibility, hallucinatory terrain, animate objects, dream (especially if the Oni personally knows your party), Chain lightning, etherealness, and many more.

I tried to include spells that fit with an oni's fluff.

Give legendary actions: like cast a cantrip, one claw attack, move up to half speed w/o opportunity attacks, (2 charges) dragons frightful presence with a reasonable DC
Only can use 3 legendary actions per round and only one at the end of another persons turn.

Then there is always the answer of MORE!
simple to add a few other baddies with him maybe? other half dragon things or giant things? maybe a vampire or some other thing that the Oni made a pact with? Devils?

[Tenmujiin]

For a campaign I'm running, I am planning on having the players encounter a half-black dragon oni (currently masquerading as an ally of the PC's) at some point. The problem; they probably won't encounter it until level 13-15, a point where a single oni is far beyond interesting anymore.

So my question is; how to buff up this oni? All help is welcome!

The easiest way would be to simply buff its health and damage until it is at the targeted CR as stated in the DMG monster creation rules.

A more appropriate change would be to make it into a legendary creature and buffing its hp/CR/AC until it is at the target CR. The second option means it will be a much more interesting fight than a CR buffed Oni and fits with it being an antagonist.

[Gwendol]

Give it class levels on top of its innate abilities.

[Mr.Moron]

Design him from the bottom up has his own thing. Forgot whatever the book says about Onis or Half-Dragons and give him the stats and abilities that fit the style and power level he's supposed to have to fulfill his role. You can certainly use the original entries as inspiration points for the overall approach but you shouldn't feel bound to them.

[ImSAMazing]

For a campaign I'm running, I am planning on having the players encounter a half-black dragon oni (currently masquerading as an ally of the PC's) at some point. The problem; they probably won't encounter it until level 13-15, a point where a single oni is far beyond interesting anymore.

So my question is; how to buff up this oni? All help is welcome!

Maybe give him a staff of ...? Or a cloack of the bat or cloack of invisibility...

[Grand Warchief]

For a campaign I'm running, I am planning on having the players encounter a half-black dragon oni (currently masquerading as an ally of the PC's) at some point. The problem; they probably won't encounter it until level 13-15, a point where a single oni is far beyond interesting anymore.

So my question is; how to buff up this oni? All help is welcome!

The problem with the players fighting a single enemy is that the players have the advantage with the economy of actions. I believe it was discussed in another thread somewhere on here, but if you buff the single enemy too much, he's going to hit one player, kill him, then die to the party. It's both not fun for the players, especially the one who just died, and not fun for you to see the encounter end so quickly. Legendary actions help, but don't really solve the problem. You really should add in more minions, or maybe have the oni bring in another oni friend. Just keep in mind the party size and how many actions they get to your badie.

Edit: http://www.giantitp.com/forums/showthread.php?404114-what-level-should-a-solo-npc-be-if-built-by-pc-rules