A bit of a silly question but having some trouble deciphering it from the rule book.
When building an encounter with multiple CRs am I reading it right that you add the CRs together to get the encounter CR?
So, say a CR 8 monster and a CR 9 monster together in an encounter would be CR 17?
No.
The exact method: add together their XP totals and compare to the total XP per encounter chart to find their CR.
The rough method: 2 creatures of the same CR = CR of each creature +2. Double the number of creatures, add 2 to the CR. 2 creatures that are 2 CR apart are equal to the higher CR of the individuals +1.
A CR 8 and CR 9 is, therefore, somewhere between CR 10 (which 9 & 7 would give) and 11 (2 CR 9 creatures)..
No, that's not right.
The easiest way to figure out the encounter CR is to combine the XP values of the creatures/NPCs involved, and compare the resulting total XP value to the CR chart.
A CR 8 creature (4,800 XP) and a CR 9 creature (6,400 XP) have a combined XP value of 11,200. That's just about halfway between CR 10 and CR 11, so your encounter is a strong CR 10, or a weak CR 11.
Another way, if you have multiple creatures of the same CR, is to add +2 to the CR for each doubling of the creatures. So, one CR 4 creature is CR 4. Two CR 4 creatures is CR 6 (one doubling, so CR 4+2). Four CR 4 creatures is CR 8, eight CR 4 creatures is CR 10, and so on.
That makes more sense. Thank you both.