Capture alive

[Hilary]

Suppose you were building encounters where the PCs had to capture the enemy alive, rather than kill them.

As a general 'average effect' what effect would this have on the difficulty?

What would the CR increase be?

[Mellack]

Are you going to allow the creatures to make death saves at zero or do they automatically die?

If you allow death saves, they just have to make sure they don't take them to negative HP or hit them again when they are down at zero. Should be simple and I would say no CR change.

If you are saying it is death at zero, they now become limited to using melee attacks to drop opponents, and that would say that is a minor CR increase.

[Unoriginal]

Suppose you were building encounters where the PCs had to capture the enemy alive, rather than kill them.

As a general 'average effect' what effect would this have on the difficulty?

What would the CR increase be?

Two things:

-CR is never affected by what the PC's goal is. Are you talking about the XP value of the whole encounter?

-As noted by other, reducing the enemy to 0 HPs with a melee attack let you knock them out. Which means the PCs just have to be careful with magic and ranged attacks, probably resulting in the fight lasting a couple rounds longer than otherwise.

[Lord Vukodlak]

Suppose you were building encounters where the PCs had to capture the enemy alive, rather than kill them.

As a general 'average effect' what effect would this have on the difficulty?

What would the CR increase be?

Depends on how much the PCs depend on damaging spells and ranged attacks.
The basic rules state
“ Knocking a Creature Out

Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.”
So for a a melee heavy party it wouldn’t make a difference.

But it might not matter much anyway. If the target is so important to the story he should follow PC death and dying rules. Which makes capturing easier.

[Vogie]

Nearly all melee weapons can be use to deal a nonleathal blow. If you absolutely want the other non-melee party members to be able to do it as well, introduce Blunted arrows as an option (as they deal bludgeoning damage, and can knock someone unconscious), and introduce wands, rods, orbs & the like with the Merciful enchantment, a la the 3.P weapon ability.

[Demonslayer666]

Suppose you were building encounters where the PCs had to capture the enemy alive, rather than kill them.

As a general 'average effect' what effect would this have on the difficulty?

What would the CR increase be?

Generally speaking, a generic combat is no more difficult if you knock them out as opposed to killing them. But it also depends on how vague the DM is about remaining hitpoints. A poorly timed arrow/fireball could result in deaths.

Are the players going in blind, or do they have some clues on how hearty these foes are? If blind, the players might opt to use melee only, and that would make it more difficult in a prolonged combat. Might be very difficult if they resist melee attacks, fly, or can disengage as a bonus action, or something similar.

If by "capture alive" you mean something similar to leading them into a cage without knocking them out, then they would have to suffer more hits leading them, so it would depend heavily on how difficult you make it to direct them.

You could also roleplay demanding their surrender (intimidation, persuasion), once you drop one, or wound some.

[Randel]

I'm thinking it also depends on what you do with them after you capture them. It would probably help if they have some manacles or rope to bind them, a cart or something to load them into to take them back, etc. Or even have some members of the guard nearby to collect the captives.

Simply knocking out your opponents instead of killing them shouldn't be that difficult. If the cleric has Spare the Dying or the druid has a handful of goodberries, it should even be possible to stabilize enemies who took lethal damage despite the player's efforts. The main issue, I imagine, would be escorting them to prison or whatnot. The easier it is for the players to turn the captives over to the local sheriff or captain of the guard to secure them, the easier it will be for them to spare opponents.