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At some point in the near future, the party are going to be facing a half-dragon enemy that at half health can transform into a full dragon (with full health).

The half-dragon has no healing abilities, so it won't be able to come back up above half health. Conditions won't carry over, concentration spells, such as Witch Bolt, will carry over. The party don't have any instant-death spells, and if they somehow manage to cleave through enough hit points to kill it before it transforms, it will then transform on death regardless. The dragon will have full HP when it appears.

The party consists of 5 Level 7s and 1 Level 5. The half-dragon is CR 5 and the dragon is CR 13 (it's an Adult White Dragon).

How do I work out the encounter XP and difficulty? Do I calculate it as two separate encounters, or is there some calculation I can do as a single encounter?

I'm not sure, but I think I've seen a question similar to this before, but I couldn't find it, so if you do see it, feel free to close this and link to it!

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This is a "Multipart Encounter"

In your case you've got two immediately-sequential encounters:

  1. A half-dragon (CR5), with only half its HP. (Because once you reduce it to half-HP it's gone! Yay! Except, "what's this?")
  2. A full-health dragon (CR13). (Crap!)

First of all, let's note that for your party (4xL7, 1xL5) even the dragon alone would be a "super-deadly" encounter. That's my term for an encounter that would fall in the "deadly" threshold even if all party members were one level more-advanced!

But when we turn to "Multipart Encounters" we get another delightful little gem:

If the adjusted XP value for the monsters in a multipart encounter is higher than one-third of the party's expected XP total for the adventuring day, the encounter is going to be tougher than the sum of its parts." (DMG p.83, emphasis mine)

Your adventuring day budget for (4xL7) + (1xL5) is 23,500 XP. (DMG p.84) This two-part encounter has an adjusted XP value of ~11,000 XP.* This two-part encounter's going to play even harder than a cake-walk followed by a super-deadly encounter. Therefore...

The dragon should easily kill your entire party.

It would have done so on its own. The half-dragon just might have tenderized the meat a bit.

* - I've halved the half-dragon's XP value, isn't quite right, but it doesn't matter in the slightest because the dragon alone is already 10,000 XP--well beyond the "one-third adventuring day" threshold!

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    \$\begingroup\$ And just to be clear, @Shem "half XP = half CR" would be a horrible rule of thumb to use, generally speaking. In this case, though, it's just fine because even if we zeroed-out the XP of the half-dragon the party'd still be in TPK-territory! \$\endgroup\$
    – nitsua60
    Commented Jan 9, 2017 at 0:15
  • \$\begingroup\$ half hp = half xp is only a first approximation. Also, sorry, I though it said it was starting at half hp. Yeah, then this will tpk the party. \$\endgroup\$
    – Shem
    Commented Jan 9, 2017 at 6:18
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I'd say just half of each XP value added together, since the party is fighting half of a half-dragon, and half of a dragon.

Don't ask me where the other quarter went.

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  • \$\begingroup\$ Huh. Didn't realize the DMG actually had that covered. Whoops. \$\endgroup\$
    – Ethereal
    Commented Jan 8, 2017 at 14:54
  • \$\begingroup\$ No worries. Ijust wondered if you'd used those rules and found them lacking in some way--that'd be really useful experience to have cataloged on the site. \$\endgroup\$
    – nitsua60
    Commented Jan 8, 2017 at 15:19
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    \$\begingroup\$ @Ethereal Apparently we misread the question, and the dragon is supposed to have full HP, not half. \$\endgroup\$
    – CodesInChaos
    Commented Jan 8, 2017 at 19:06

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