The party is in Rise of Tiamat and their next encounter will be
The Cult Strikes Back, Third Attack
I am trying to budget for a Deadly encounter, as suggested in the adventure. One of the possible foes to include is a half-green dragon assassin.
As far as I am aware, there is no official stat block for this monster; it is not in Appendix A of Rise of Tiamat, but a stat block is provided in the Roll20 version of the adventure (which we are using).
The table given for the encounter, both in the text and Roll20 versions of the adventure, lists the XP value of the half-green dragon assassin as 3900, which would correspond to a CR of 8, equal to that of an Assassin.
The text version of the adventure includes a footnote which states "Use the assassin statistics but add the breath weapon of a green dragon wyrmling. This change doesn’t affect the assassin’s challenge."
The Roll20 version of the adventure does not include this footnote, and further links to a stat block for the half-green dragon assassin that lists it at CR9 with an xp value of 5000. The 1100 point difference could be the addition or subtraction of a dragonfang or three dragonwings to or from the encounter.
While there is an errata for Hoard of the Dragon Queen, as far as I can tell there is no such errata for Rise of Tiamat.
Recognizing that official creature stat blocks can have CRs that are not those that would be derived from the DMG calculation method, is there anything to recommend treating the half-green dragon assassin as either CR 8 or 9? Does its one additional ability,
Poison Breath Recharge 5-6
The dragon exhales poisonous gas in a 15-foot cone. Each creature in that area must make a DC 11 Constitution saving throw, taking 21 (6d6) poison damage on a failed save, or half as much damage on a successful one.
argue for an increase in the CR above that of a standard assassin?
The party is five PC's, as well as a bat familiar, imp Chain Master familiar, and a Shield Guardian, as well as an NPC
whose primary focus will be on protecting herself, then defeating the attacking mage, but with little regard for the other cultists or protecting the party.
Update: I am now realizing that the Dragonfang, listed in Appendix A of Rise of Tiamat text as a CR5 foe, has an XP value in the encounter table of 1100, corresponding to a CR4. Thus, I am also unsure of whether I am interpreting the purpose of this table correctly.