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How do I calculate the challenge rating of monsters that feature abilities that do not directly do damage?

For example, my homebrew monster has the two following abilities:

The target takes a Constitution saving throw. On a failure, it drops to 0 hit points.

This one could be used on a character with 10 hit points, or 100, which creates drastically different damage results depending on character level.

All creatures within 60ft take a Wisdom saving throw. Each target that fails is stunned for 1 round.

This can be extremely dangerous if this monster is paired with monsters that can make use of the stun condition, but it is virtually useless if the monster is alone.

I'm sure there are infinitely more examples of this type of thing.

How do we calculate CR for monsters that do not rely on standard attacks?

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Convert it into a statistic which can be quantified, then playtest it

According to the monster creation rules, Dungeon Master's Guide p.273-283, monster CR can only truly be calculated by playtesting. Page 275 notes:

Creating the monster isn't just a number-crunching exercise. The guidelines in this chapter can help you create monsters, but the only way you can know whether is fun is to playtest it. After seeing your monster in action, you might want to adjust the challenge rating up or down based on your experiences.

However, guidelines appear in the Monster Features table, p. 280-281, which may help to benchmark it. This table can be summed up as so: a monster trait can effectively increase the monster's AC, HP or per-round damage for the purpose of CR calculation, based on the equivalent amount of offensive or defensive strength it gains.

In particular:

  • The banshee's Horrifying Visage is an area effect which deals no damage but effectively makes the banshee less likely to take hits due to opponents being frightened. It increases the monster's hit points by 25% for the purpose of CR calculation.
  • Dealing instant-kill is effectively dealing a lot of damage on a single target. I can't find an equivalent in the table, but you might want to compare the CR6 medusa to the CR the algorithm says she would have without the Petrifying Gaze.
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  • \$\begingroup\$ You could also consider average HP of PCs on different levels. And since DM can somewhat guess at what level are the PCs going to be in that encounter, this gives you a good estimate of the damage. Even more so, since you know what classes your party consists of. Your average party member will have d8 or d10 hit dice, so it does around [LVL]d8 to [LVL]d10 + [LVL]*CON damage on a failed save. Nice thing about this is that the CON save goes against the damage, i.e. high CON means technically more damage, but also better chance of saving against it. \$\endgroup\$ – J.E Oct 7 '19 at 9:06
  • \$\begingroup\$ Another thing you could consider is how high the spell save DC is. The higher the save DC on the CON save is the more difficult it would be to pass unless the players were higher level. A DC 11-12 save is slightly above average and doable, but a DC 15+ is going to be considerably harder unless the PC is proficient. \$\endgroup\$ – mullac Oct 8 '19 at 0:56

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