Howdy! As per a post I made a couple of days ago, I've just written up a CR17 Headless Horseman to fit the lore in my homebrew world of an Autumn Eladrin being executed and now the land surrounding is cursed with the Horseman. I need it to be both undead and fey feeling to give a challenge to a party of super synergetic players. 

Please have a look and tell me what works and what doesn't. I smooshed together elements of a Death Knight, a Autumn Eladrin and some other bits from the Endless Road module from Dragon+. There's a lot of fluff/text but thats mainly for my own retentive benefit.

Version 1: https://www.dndbeyond.com/monsters/1222964-the-headless-horseman

Version 2: https://www.dndbeyond.com/monsters/1230163-the-headless-horseman

Version 3: https://www.dndbeyond.com/monsters/1230499-the-headless-horseman

CR is probably a bit off. You seem to have basically tweaked a Death Knight, but you added a +3 weapon, Legendary actions (which double its damage output), Legendary resistance, resistance to normal weapons. It does have a bit lower defense due to inferior AC.

If you want to keep it at CR 17, I would:

• Reduce multiattack to 2 attacks.
• Reprice legendary abilities; I would make Attack cost 2 actions, and the other three all cost 1.

If you aren't actually after CR 17 I'd probably still switch off some offense for defense.

Quote from Pantagruel666 >>

CR is probably a bit off. You seem to have basically tweaked a Death Knight, but you added a +3 weapon, Legendary actions (which double its damage output), Legendary resistance, resistance to normal weapons. It does have a bit lower defense due to inferior AC.

If you want to keep it at CR 17, I would:

• Reduce multiattack to 2 attacks.
• Reprice legendary abilities; I would make Attack cost 2 actions, and the other three all cost 1.

If you aren't actually after CR 17 I'd probably still switch off some offense for defense.

 Thank you very much! I have amended those things now! I appreciate that.

You might also want to specify, in the Fey Step ability, if it provokes an attack of opportunity or not. If it does, he's going to get hit every time he uses it within someone's melee reach... if not it is a good way to get out of melee range if he wants to get some space for some reason. Ordinarily unless an ability says it doesn't provoke an AoO, it will if you move out of someone's melee range.

Under "Born in the Saddle", you refer to "you" instead of the Headless Horseman. Also, do you know how powerful Plane Shift is? Look at the last paragraph of the spell.

I might be wary of stacking magic resistance with legendary auto-success on saves. It might make your casters feel a little too penned in if they will basically never land one of their save spells. Always be wary of making a player feel useless in a boss-fight.

Odd Wording -  "If The Headless Horseman reaches dies in battle, Spitfire will use its reaction to flee using its Ethereal Stride action, otherwise it will do so on its next turn. When Spitfire reaches dies, The Headless Horseman will stand and fight to the death." 

Link Error -   Share the Trophies (1/Day). The Headless Horseman summons 4 (1d6+1) [Tooltip Not Found] 

I would consider giving resistance to BPS, even magical. 

Magic Resistance is fine, he's the BBEG.

Quote from BioWizard >>

You might also want to specify, in the Fey Step ability, if it provokes an attack of opportunity or not. If it does, he's going to get hit every time he uses it within someone's melee reach... if not it is a good way to get out of melee range if he wants to get some space for some reason. Ordinarily unless an ability says it doesn't provoke an AoO, it will if you move out of someone's melee range.

 Very true! Thank you very much! I will make that amendment!!

Quote from Thauraeln_The_Bold >>

Under "Born in the Saddle", you refer to "you" instead of the Headless Horseman. Also, do you know how powerful Plane Shift is? Look at the last paragraph of the spell.

 Oh oops! I took that ability trait directly from the Cavalier fighter subclass and obviously missed that one! Thank you!

Yeah, I kinda took that into consideration and my logic was that he'd only ever transport the target to a spot of the Feywild's Endless Road IF he used that part of the spell. Im my head, he'd only be using that to switch between the Material Plane and the Feywild himself. BUT now I've just realised naturally that other people may (probably wont) use this too. SO, I'll amend that spell to read "At will: plane shift (self only)". Thank you very much for picking me up on that! 

Quote from Teh_Professor_96 >>

I might be wary of stacking magic resistance with legendary auto-success on saves. It might make your casters feel a little too penned in if they will basically never land one of their save spells. Always be wary of making a player feel useless in a boss-fight.

 Truth! I hadn't thought of that! Essentially that was a carry over from Death Knight stat block and hadn't realised the interactions with the bits I've added. Thank you!!!

Quote from Wysperra >>

Odd Wording -  "If The Headless Horseman reaches dies in battle, Spitfire will use its reaction to flee using its Ethereal Stride action, otherwise it will do so on its next turn. When Spitfire reaches dies, The Headless Horseman will stand and fight to the death." 

Link Error -   Share the Trophies (1/Day). The Headless Horseman summons 4 (1d6+1) [Tooltip Not Found] 

I would consider giving resistance to BPS, even magical. 

Magic Resistance is fine, he's the BBEG.

 Oh dang! Both of those bits you've highlight in green were originally written as 'reaches 0hp, but then I decided to change it (because my cleric has done dumb things in the past like healing/reviving monsters so as to keep wailing on them haha) and failed to delete 'reaches'. Thank you for the pick up.

I'll meddle with that tool tip then and get it linking to that creature. 

I thought about that, but I didn't want him to be TOO powerful as I want him to be an encounter a day boss, something to run away from or be wary of. BUT all it takes is for one person to urge me and ITS BACK IN THE STAT BLOCK! hahaha thank you!

So further more, apologies for my many mistakes and oversights in this stats block, I threw it together over a couple of days during short, regularly interrupted breaks at work and over the nightly tv binge with the wife, so I wasn't completely focused in its creation. I appreciate the in put greatly. Keep it coming.

Updated with a new link in the top comment. You were all right, it looks much better now.

Using his gust of wind Lair Action along with Ride By, the Headless Horseman can effectively make opportunity attacks against everyone who fails their save, and can do this every other round. You could choose to use this as a tactic if you want to.

Also, by casting grasping vine  as a bonus action, the Headless Horseman can achieve the same result, while also getting them into a cluster for Hellfire Orb.