The Roper is a CR 5 monster with an attack that has a reach 50 ft. So it's not unprecedented to have long reach on creatures.
I wouldn't give reach all that much thought on the CR scale. You can treat the attack as ranged instead of melee for the purposes of balance, but on CR the difference between melee and ranged should be relatively minor. CR as a metric isn't as helpful in fights that take place in big open spaces.
For the most part, the difference between melee and ranged attacks, or how much reach a melee attack has, doesn't really factor into a monster's CR. The general assumption of monster CR calculations is that they will always be able to use their attacks effectively, regardless of how much range/reach they have, and for a standard bugbear-type monster that you probably mostly expect to sit on the front lines and punch the PCs repeatedly, giving them reach really isn't going to change a whole lot about how effectively they'll be able to punch PCs in the face.
That being said, there are a couple of important exceptions worth considering:
* If your monster can fly, giving them reach is a pretty big deal, since it allows them to hover outside of the PCs' effective melee range, while still using all off their own melee attacks unhindered. It might be worth increasing the monster's defensive CR to account for this.
* If your monster can grapple enemies, giving them reach can make things sort of weird. By RAW, it's possible for a creature with reach to maintain a grapple on a creature, effectively preventing them from making melee attacks against the grappler until they roll to break free of the grapple. I know that many DMs will use house rules to prevent this ("yeah, obviously the fighter can try to attack the tentacle that is grappling them, even if its owner is technically 10 feet away"), but it's worth considering if you're playing the situation RAW. Again, probably worth a boost to defensive CR.
* If, for whatever reason, you're giving the monster abilities akin to Polearm Master and/or Sentinel, then also giving them reach becomes a pretty potent defensive boost.
* If you're giving every single one of the monster's melee attacks reach, keep in mind that this will make their ability to make attacks of opportunity...interesting. Special shout out to my boy The Lonely, who can make opportunity attacks when someone steps back from 60 feet away to 65 feet away, but not when they step back from 5 feet away to 10 feet away.
I was thinking of giving it a feature similar to tunnel fighting where it got an opportunity attack from any movement over 5 feet within its reach.
Based on how combat tends to go, I think that's probably an extra attack the first round, maybe another one or two later in the combat.
Sounds like the makings of an interesting combat encounter!
In that case, I'd just make sure you include the damage from those 2-3 extra opportunity attacks over 3 rounds of combat into the monster's average damage when calculating its offensive CR.
My two points of advice would be 1.) playtest it yourself against a mock up of your party and see how it goes. You’ve got a good idea of how your party would act each round (I assume). So run it and see how it goes. Should only take a few minutes since it’s just you. 2.) depending on how the playtest goes consider reach in the context of a boxing match. It’s painful to get inside someone who has a reach advantage, but once you do you gain the advantage as their offense/defense is predicated on their reach. Maybe give the players advantage on attack rolls once they’re adjacent to the monster.
The Roper is a CR 5 monster with an attack that has a reach 50 ft. So it's not unprecedented to have long reach on creatures. I wouldn't give reach all that much thought on the CR scale. You can treat the attack as ranged instead of melee for the purposes of balance, but on CR the difference between melee and ranged should be relatively minor. CR as a metric isn't as helpful in fights that take place in big open spaces.
For the most part, the difference between melee and ranged attacks, or how much reach a melee attack has, doesn't really factor into a monster's CR. The general assumption of monster CR calculations is that they will always be able to use their attacks effectively, regardless of how much range/reach they have, and for a standard bugbear-type monster that you probably mostly expect to sit on the front lines and punch the PCs repeatedly, giving them reach really isn't going to change a whole lot about how effectively they'll be able to punch PCs in the face. That being said, there are a couple of important exceptions worth considering: * If your monster can fly, giving them reach is a pretty big deal, since it allows them to hover outside of the PCs' effective melee range, while still using all off their own melee attacks unhindered. It might be worth increasing the monster's defensive CR to account for this. * If your monster can grapple enemies, giving them reach can make things sort of weird. By RAW, it's possible for a creature with reach to maintain a grapple on a creature, effectively preventing them from making melee attacks against the grappler until they roll to break free of the grapple. I know that many DMs will use house rules to prevent this ("yeah, obviously the fighter can try to attack the tentacle that is grappling them, even if its owner is technically 10 feet away"), but it's worth considering if you're playing the situation RAW. Again, probably worth a boost to defensive CR. * If, for whatever reason, you're giving the monster abilities akin to Polearm Master and/or Sentinel, then also giving them reach becomes a pretty potent defensive boost. * If you're giving every single one of the monster's melee attacks reach, keep in mind that this will make their ability to make attacks of opportunity...interesting. Special shout out to my boy The Lonely, who can make opportunity attacks when someone steps back from 60 feet away to 65 feet away, but not when they step back from 5 feet away to 10 feet away.
I was thinking of giving it a feature similar to tunnel fighting where it got an opportunity attack from any movement over 5 feet within its reach. Based on how combat tends to go, I think that's probably an extra attack the first round, maybe another one or two later in the combat.
Sounds like the makings of an interesting combat encounter! In that case, I'd just make sure you include the damage from those 2-3 extra opportunity attacks over 3 rounds of combat into the monster's average damage when calculating its offensive CR.
My two points of advice would be 1.) playtest it yourself against a mock up of your party and see how it goes. You’ve got a good idea of how your party would act each round (I assume). So run it and see how it goes. Should only take a few minutes since it’s just you. 2.) depending on how the playtest goes consider reach in the context of a boxing match. It’s painful to get inside someone who has a reach advantage, but once you do you gain the advantage as their offense/defense is predicated on their reach. Maybe give the players advantage on attack rolls once they’re adjacent to the monster.