[RPG] How does the summoning ability affect the encounter difficulty

cr-calculationdnd-5eencounter-designsummoning

The Drow Mage (CR 7) has the Action Summon Demon (1/day), which allows him to summon a shadow demon (CR 4). Is the shadow demon already included in the CR of the Drow Mage, or do I have to count the shadow demon as an extra creature to determine the difficulty of the encounter?

Best Answer

Probably the summoned monster doesn't count toward the challenge.

Bear in mind there's no RAW that specifies this, and challenge ratings of published monsters include some degree of "eyeballing" from the writers. But we can extrapolate a bit by using the basic challenge evaluation from the DMG on the stats that we do know.

I used Monstermancer* to run the stats of the Drow Mage without the summoning ability, counting all of its spells, and came up with a baseline challenge of 7:

"eval": {
    "defCR": 0.5,
    "offCR": 15,
    "dmg": 126,
    "ac": 15,
    "hp": 45,
    "atk": 6,
    "svDC": 14
}

So, without any eyeballing or special tweaks to its challenge evaluation, the Drow Mage's challenge matches the one given in the Monster Manual (in fact, this might be a little low, since it doesn't factor in the possibility of having an ongoing cloudkill spell active while throwing lightning bolts, but I digress).

However, while it looks like the summoned creature isn't "baked in" to the summoner's challenge rating, there isn't much guidance as to how you should incorporate it into an encounter. It's less effective than a full-fledged combatant for a few reasons (it takes an action to summon, it might not always succeed, &c.), but adding a challenge 4 shadow demon to a fight against a single challenge 7 mage is not a minor impact on the encounter, so it should count for something.

For a little more circumstantial evidence, there are variants listed in the Monster Manual for demons and devils which can summon other demons and devils; there is an entry for each type of fiend and which others they can summon with a percentage chance, similar to the ability of the drow mage cited here. These variants don't mention a change to the fiends' challenge ratings. Since the book doesn't assume a difference in the creature's own challenge with or without the summon ability there, it follows that the summoned creature should count separately in some way (even if not as a full-fledged combatant).

*Caveat: Monstermancer isn't perfect at this stuff, but it's an OK starting point. Also, there's a huge mismatch between the offensive and defensive challenges it gives to the Drow Mage, so it's a lot easier to take out than its challenge rating implies.

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