The boss alone is amazing, genuinely really fun to fight
However his summons makes him tougher than necessary, but you can use your own summons so there's that.
And bewitching branches will make the fight even easier.

This guy caused me hours of pain on the first run though. NG+ with a couple of serious weapons and I took him on the 5th go with no summons. Finished with a crit which was very enjoyable: )

Hard as hell. Dex build barely beat him at lvl 120. Unsheathe on dual wield, roll back, unsheath again then he lost posture, easy kill. Boss and shield minion were beating the shxxxt out of my spirit ashe. Tried another unsheathe to hit both, rollback, charge in, took down shield minion. Boss AOE and tried to jump attack on me and got frost bite. Then Boss went kneel down for a long time, landed 3 or 4 hits on him. 4th unsheathe got him stun but my spirit ashe died right after. Now boss had 1/3 hp left. Running in circles and bait for his jump or charge attack then roll behind him and hit him 1 or 2 twice while keeping hp above 80% all the time. Last hit was another unsheathe right on his head while he was about to do another AOE. used 7 out of 11 flasks of crimson tears.

The bewitching branch works, I've used it but only from an unseen approach. Some of the comments here make it sound like you can stroll up to the knight powerstancing greatswords who'll wait politely for you to poke him with a branch while Niall and the other one have a chat about the football.
That wasn't my experience in my first NG+ no, instead all three were battering me at the same time, they killed me three times very quickly before I stopped trying and equipped ekzykes's decay. That did work dissolved the spectral nights in one cast and proc'd Niall, good beans. Also, 'waves of darkness' on a +25 zweihander works very well against commander Niall. It isn't difficult to catch him with all three waves and the optional follow up, with a weapon like zweihander that follow up attack does impressive damage, with the possibility of doing even more if you cast blackflame blade on the sword before using WoD all three waves slamming down take on the DoT characteristics of the blackflame.

Honestly this guy has one of the most stylish moves I've seen with his halberd combos and wind ups, hes a pretty cool boss

Each attack has a giant AoE ✓
No openings ✓
Frost buildup ✓
Lightning (why not) ✓
Jump attack with a range of a whole arena ✓
Tanky ✓
Deals 3x damage of a previous location story boss ✓
Is a summoner ✓
You can't reach a legacy dungeon without fighting him ✓

Truly one of the boss fights fromsoftware ever created.

this is the worst boss in the ****ing game

Aced the fight with the ol' "tower shield and poke" strategy. Ran in, guard-poked the dual wielder down, then killed the shield guy with a mix of pokes and guard-counters. Niall himself wasn't so bad on his own. He can suffer from bleed, rot, and poison so use those consumables if you don't have it on your weapon already.

Beat him on the first try at level 156 or so. Summoned Greatshield Soldiers for crowd control. The spectral knights were easy to take down, they were pretty weak actually. Then the Greatshield Soldiers distracted him long enough for me to do the magic thing.

Believe the hype about bewitching branch trivializing this guy. It can be hard to land b/c the wind-up is slow, but if you can poise through it and get the 2x sword knight charmed, he will murk the other knight and severely chunk Niall. Add your own spirit ash (Ancient Dragon Knight Kristoff is great for tanking if you're ranged) if you want to turn the tables and make it a gank against Niall.
Niall will murder that bewitched knight pretty quick, but he'll do an absolute pile of damage before he goes down.