Testing Tiamat

[Eldariel]

Those are both basically yes/no question spells, given that you want to also do summoning and binding in the run up to the fight I don't think spamming these is all that feasible so what yes/no questions would you ask to get to LMI without any meta knowledge/logic?

Well, COP is "One-word answer or short phrase if one-word answer would be misleading". The usual question battery could be e.g.:
- What is the strongest enemy we shall face on the coming day?
- What is their most troublesome immunity to weapon users? [E.g. nonmagical weapon damage immunity]
- [Optionally go through all elements] Are they immune to [Element]?
- Are they immune to magic spells of some level?
- [If yes] What is the lowest spell level that works on them? ["None" would imply a Golem-type total magic immunity while otherwise you could figure out the level of spells that works here]
- What other immunity of note do they have?
- What is their most dangerous attack?
- What is the strongest spell they can cast?

You could formulate many of those in a way that works with Commune:
- Is the strongest enemy we shall fight on [day X] immune to X?

Go through all important immunities and resistances. Then "Can they cast spells?", "Do they have area of effect damage?", etc. can be used to gain further information. As long as you can identify the most powerful enemy, this is applicable to most types of boss-level encounters. Of course, you can adjust questions on the fly depending on enemy AC. Between 6 Portents, there's a reasonable chance of getting enough for 3-4 castings of COP which is 15-20 questions.


EDIT: I rolled the-day-before Portents in the roll thread. 1 and 7 guarantee Planar Bindings and 11-19 guarantee 4 successful Contact Other Planes so I could ask 20 questions. Further, I can ritualize Commune on the last day and this day so there's 6 accurate Commune questions available.

EDIT#2: Preliminary draft of the character sheets done minus equipment. PHB only is making some buffing issues more apparent since the spell lists of especially Druids and Clerics are much more limited in a PHB only settings but it seems workable, if a bit rough. I'm still pondering few options for the Bard (Counterspell vs. Wall of Force) and some Druid/Cleric choices as well as how to apply the rest of the long term buffs and how many slots to put into buffing in general, but that's largely whatever level. I've probably put some useless spells on the lists and forgotten some of the essential ones: this is the issue with doing stuff out of memory and not developing character over a campaign (FWIW I've used some pre-existing spell lists as a blueprint but aside from the Swords Bard and the Diviner, I haven't played these exact subclasses with this kind of party on this level so the needs are a bit different and thus the party dynamics change too).