Help me understand how the hit points work.
Baseline:
Summoner: 14 Con
Eidolon: 13 Con
Lvl 1
1. With the above baseline, the following is true?
Summoner has 10 hit points when not fused.
When fused the Summoner has 9 hit points and the Eidolon has 6 hit points for a total of 15 hit points combined.
My summoner personal pool of hit points drop when fused?
2. If I add a +2 enhancement bonus to my Con from a magic item, the following is true?
Fused Synthesist:
Summoner has 10 hit points (8 from HD, +1 from 13 Eidolon Con, +1 from enhancement bonus) and the eidolon has 7 (5 from HD, +1 from Con, +1 from enhancement bonus from Con). 17 total hit points.
3. Synthesist gains large size from evolution surge spell gaining +4 size bonus to Con.
Summoner has 11 hit points (8 from HD, +1 eidolon con, +2 from size bonus to Con) and the eidolon has 8 (5 from HD, +1 con, +2 size bonus to Con). Total of 19 hit points.
The following is true:
The summoner's hit points drop when fused if Con is higher than the eidolon's Con?
Summoner hit points and eidolon temporary hit points increase when the eidolon's Con increases from enhancement or size bonuses?
Is this how it works? You have to track all this depending on the circumstances?
I am surprised you found a GM willing to allow one of these.
1.)
Synthesist (unfused) is a d8+2 so at first level its 10 hp (excluding favoured class bonus of course)
Synthesist (fused) is d8+1..so 9 hp. Eidolon normally has d10 hp but since they are never taken as full at first level, it is half of 10=5+1=6. But these are not hp. there are temporary hp. If the temporary hp ever fall below 1 the synthesist becomes unfused.
So yes.. the hp change when you fuse. I suggest having the same con modifier before and after fusing to save loads of maths.
2.)
Fused synthesist indeed gets 10 normal hp and 7 temporary hp. Hooray for doubling up on hp per Con modifier.
3.)
Fused synthesist indeed gets 11 normal hp and 8 temporary hp. Hooray for doubling up on hp per Con modifier again.
The summoner's hit points drop when fused if Con is higher than the eidolon's Con?
So his total hp does drop but he also gains a pool of special temporary hp so in effect it kinda increases.
Summoner hit points and eidolon temporary hit points increase when the eidolon's Con increases from enhancement or size bonuses?
True. It's this double-up on hp and temp hp that makes the synthesist (among other things) a little bit broken.
Is this how it works? You have to track all this depending on the circumstances? Yes that is how it works. It goes even further though since the temp hp do not heal naturally so they have to be tracked even when the "eidolon armor" is unsummoned.
For a more balanced take on the synthesist take a look here
The healing part I have worked out. Rejuvenate eidolon and life conduit are two spells specifically meant to heal an eidolon. I'm assuming those are the only options to get the temporary hit points back. I figured I can use wands for dealing with that.
We've never had anyone play a synthesist. If it proves to be too strong, we'll adjust.
Yes. The rejuvenate eidolon spells are effectively a spell tax for the synthesist. And wands of cure X wounds will only help for the normal hit points no the temporary hits points.
You could just get a Wand of Lesser Rejuvinate Eidolon for the same price as a CLW wand.