I'm thinking about creating a character (Bugbear) with a "Reach" weapon, to give me a 15' reach. Is there a Feat which will give me an Opportunity Attack, once an enemy enters that range? What are the other benefits and drawbacks? Thanks

Polearm Master

Benefits: when you bonk or stabbystab a peep with your Action, you can extra bonk with a bonus action for 1d4 bonkiness.

Spicy Benefit: When the fools enter your bonk/stabbystab zone you can make an oppy attack.

Drawbacks: zilch. Squat. Natta. 

Season with Sentinel for extra super spicy bonkage.

I haven't slept. Brain not good. Need sleep. Or therapist.

Thanks Cyb3rM1nd!

Go with the cheaper (sleep) option first!

 Oh, and I am going with the "Pike" because I need the "piercing" property for the "Piercer" feat. I can't find much in the way of 2-handed weapons (non reach) with the "Piercing" property. It would be a nice backup weapon. Spear maybe?

Keep in mind that the bugbear extra reach only applies to attacks on your turn, so opportunity attacks are at normal range. And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'.

There is also Cavalier Fighter if you want infinite OAs, allows you to hit them if they try to move or leave while within your reach, and reduces their speed to zero if they got hit. PAM feat lets you hit them when they first enter your reach. Sentinel feat lets you hit disengaged enemies. Blind Fighting fighting style lets you hit invisible enemies. If you want some extra accuracy, you can take Mounted Combatant feat to get advantage, and Elven Accuracy feat to further boost that advantage if you do not mind changing your species. For elves, I prefer Eladrin since their Fey Step have extra effects that help you with this role: winter lets you Frighten enemies away from you (and by extension your allies), while spring lets you teleport allies next to you further back away from danger. Legacy Eladrin lets you recharge Fey Step on a short rest, while modern Eladrin recharges Fey Step on a long rest, but you can spam Fey Step up to your PB.

I personally lean towards glaive/halberd over pike when taking feats into account, since I think Slasher's movement reduction is more relevant than Piercer's extra damage if your role is to lock things down and have enemies stick to you. Zero speed from Hold the Line/Sentinel only lasts until the end of the current turn, while Slasher's movement reduction lasts until the start of your next turn, so if an enemy manages to slip past you, it may take a little longer for them to reach your allies behind you.

I do not think Warcaster feat works with Cavalier Fighter's infinite OAs, since Vigilant Defender specifically states that the infinite OAs can ONLY be used for OAs, but you can ask your GM to see if they will allow replacing OAs with spells. If your GM allows that combo, you can use Booming Blade to give enemies the choice of moving out of your reach at the cost of taking extra damage.

Quote from _Amunsol_ >>

Keep in mind that the bugbear extra reach only applies to attacks on your turn, so opportunity attacks are at normal range. And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'.

 
"the bugbear extra reach only applies to attacks on your turn"

Dis do be true. Makes no sense. Why does arm length change on turns? Arm length be arm length. O.o Ah well. Weird as fluff StretchPowers away!

I wanna make a joke about stretching makes ya limber (geddit, stretch... limb...) but I'm not sure if it would have quite the right reach.

I still haven't slept, OK, this the best my glitchy brain can conjure.

"And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'."

Dis be LIES 👿👿👿

I kid 😂. Kinda... 👀 So there is no specification of range difference for the bonus bonk. It uses your normal my-lady melee reach which is 10 ft due to the long weapons and extra 5 (totes 15 ft) due to being furry mr fantastic.

Quote from dragonflight777 >>

Go with the cheaper (sleep) option first!

 
On checking my bank account I haz come to find your suggestion most wise. I needz naps. 

Quote from Cyb3rM1nd >>

Polearm Master

Benefits: when you bonk or stabbystab a peep with your Action, you can extra bonk with a bonus action for 1d4 bonkiness.

Spicy Benefit: When the fools enter your bonk/stabbystab zone you can make an oppy attack.

Drawbacks: zilch. Squat. Natta. 

Season with Sentinel for extra super spicy bonkage.

 Also, glaives just look incredibly cool. All the mechanical stuff is nice - but looking super extra awesome in combat is like free crits all the time (except not in the mechanical sense).

Quote from Cyb3rM1nd >>

Quote from _Amunsol_ >>

Keep in mind that the bugbear extra reach only applies to attacks on your turn, so opportunity attacks are at normal range. And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'.

 
"the bugbear extra reach only applies to attacks on your turn"

Dis do be true. Makes no sense. Why does arm length change on turns? Arm length be arm length. O.o Ah well. Weird as fluff StretchPowers away!

I wanna make a joke about stretching makes ya limber (geddit, stretch... limb...) but I'm not sure if it would have quite the right reach.

I still haven't slept, OK, this the best my glitchy brain can conjure.

"And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'."

Dis be LIES 👿👿👿

I kid 😂. Kinda... 👀 So there is no specification of range difference for the bonus bonk. It uses your normal my-lady melee reach which is 10 ft due to the long weapons and extra 5 (totes 15 ft) due to being furry mr fantastic.

Quote from dragonflight777 >>

Go with the cheaper (sleep) option first!

 
On checking my bank account I haz come to find your suggestion most wise. I needz naps. 

 pfft sleepy cybermind makes all the fun words, but this is correct

I'd add in a grab for eldritch claw though (if you at all can in the campaign), if you dont want to take polearms and cavalier fighter and sentinal

So, I would get an OA with POLEARM MASTER"

• While you are wielding a glaive, halberd, pike, quarterstaff, or spear, other creatures provoke an opportunity attack from you when they enter your reach.

 ..and then SENTINEL:

• When you hit a creature with an opportunity attack, the creature's speed becomes 0 for the rest of the turn.

It might not be the "best" Fighter build, but I think I can have fun with the Rune Knight.

Quote from Cyb3rM1nd>>

"And I believe the polearm master bonus attack is also shorter than the normal polearm reach, 5' instead of 10'."

Dis be LIES 👿👿👿

I kid 😂. Kinda... 👀 So there is no specification of range difference for the bonus bonk. It uses your normal my-lady melee reach which is 10 ft due to the long weapons and extra 5 (totes 15 ft) due to being furry mr fantastic.

 Hmm, I believe you are correct. I came to think that because on the DDB character sheet under actions the polearm master bonus attack is listed as being shorter. Maybe it has something to do with the coding for the attack feature not synching with the actual weapon used?

the 15' is for attacking with a polearm (or other reach weapon), and the polearm master feat gives you that opportunity attack with a polearm (or speak or staff) when they enter 10' of your range (not 15). If you had the Sentinel feat as well, it would stop their movement at 10' when your opportunity attack hits.

Is the Bugbear has a "Reach" of 5', I would think it would be added to the polearm reach. 

Only for attacking, not for defense with polearm master's opportunity attacks....that is still 10'. The wording for the Bugbear ability, long limbed, is very clear...a melee attack on your turn gains 5'.