Ok, recently i got this idea for a build that can shut down melee fighters:

Bugbear Cavalier 18/Warlock 2 with War caster feat, Repelling Blast invocation, Eldritch Blast, and a weapon with reach.

How it works

Cavaliers 18-level ability basically allows you to get an opportunity attack on every creatures turn. This will allow you to attack every creature that comes within 15 feet of you due to your reach weapon and bugbears Extended Limbs. Combined with Hold The Line that reduces your speed to 0 feet when hit by an OA and Eldritch Blast + Repelling Blast due to War Caster, which will push them back 10 feet, then they can't touch you unless they have 25 feet reach. Of course this is all useless if their attacks are ranged

PS. Is this build even RAW?

Second PS. Sorry if i have written it badly

Third PS. Is this even the right forum to post this?

Pretty sure using Warcaster replaces the AoO, and thus you don't get the benefits of hitting a target with a AoO because your not. You're hitting them with a Warcaster instead.

Might be wrong though.

Lets poke some holes!

First: 4yulming4 is right, warcasters spell (eldritch blast in this case) no longer counts as an AoO - It's just a spell(not a warcaster, just a spell).
Second: The bugbears long limbed doesn't work on other creatures turns. It specifies that it only works on your turns. While the enemy could theoretically move on your turn via some ability, it normally dosesn't work on AoOs.
Third: this build assumes lvl 20 , which means you're supposed to have broken abilities. This ain't that broken for the investment. Onestly Sentinel Polearmmaster works almost the same in most cases.

I always miss something when i make these kind of builds

You seem very excited to poke holes Sutio lol.

I can't say much either I jumped pretty quickly to poking holes too so.

Build is a very cool idea, I think replacing Warcaster with Crusher could do something similar. No Warlock levels needed, but you can only push 5 ft instead of like 40, and it doesn't work on large or larger targets.

Edit: not quite as insane as the OP, but PAM + (Sentinel/Hold the Line) + Crusher is a very respectable combo.

Yea poking holes is kinda fun ;)

Crusher would be cool on this kind of build, but doesn't work RAW since there is no weapon that deals bludgeoning damage and has the reach property.

A Ten Foot Pole wielded as an improvised weapon with proficiency via Tavern Brawler might be a RAW-adjacent way to get bludgeoning at Reach. Or just make puppy dog eyes and ask your DM to let you swat people with the flat of your Halberd, or even blunt it entirely to turn it into more of a pole hammer.