So say you're a synthesist. Massive number of attack, + to hit etc. If you have any summoningtype caster on your team you can destroy everything without fear. Have you team mate summon an invisible stalker, have it allow you to possess it, profit. If you get defeated the stalker's body disappears and you go back to yours safe and sound. With communicate with animals you could possess a summoned dire tiger and get all of its natural attacks & yours.
I'm about to run a game and one of the players said that he might go synthesist and that got me thinking about worst case scenarios...
It doesn't quite work that way, and there is a number of reasons why I don't think it can be exploited (in that way).
1) It is debateable whether you actually can fuse with the eidolon in another body. You probably can, but it is a matter of GM interpretation on whether it is a purely mental ability, or it is dependant on your own body (having the glowing rune in your forehead).
2) Even if you can merge with the eidolon in another form, it is not going to do you much good. When you do so, you'll take on the ability scores and limbs/natural attacks of the eidolon. So you don't get more natural attacks, as a tiger for example.
3) For the invisible stalker, it requires him to be able to see it, if he is going to target it with marionette possession.
4) As far as summoned monsters go, duration is a huge limitation. Since they are going to stay for 1/lvl, and the ritual to summon the eidolon takes 1 minute, he is probably limited to using summon eidolon to do so. Even then it is going to take up a couple of action in the start of an encounter. The wizard (or other caster) has to summoned. Unless there is special circumstances, the syntesist is waiting (or casting a buff spell) until the summoned creatures arrives the next turn. Then he spend his second round possessing the creature (if he can see it), and the third round merging with the eidolon, and is ready to fight in the fourth round. Not very efficient, compared to being a part of the battle from the get-go.
Well that's a clever idea...
I don't think he'd be totally invisible as the stalker. (eidolon forms a translucent shell, so mostly visible) As for the tiger, no matter the body it's surrounding, the eidolon is still limited to the number of natural attacks it can make for it's level. It can choose to use it's own or the tigers but not both at once. If you play a kitsune with a bite attack the eidolon doesn't get an extra attack either.
Intelligent enemies, after fighting off a random tiger that disappeared when defeated, could put 2+2 together and realize there's a summoner nearby. With the distance limit on the possession that would make pulling it off more than once very risky. Alternatively, if you're attempting this in the middle of a battle, tossing an axe or two at the helpless summoner would be mighty tempting.
EDIT: sneaky ninja
Lvl 10 elf can summon the eidolon in 1 round, so summoning time can/could be mitigated. For targeting the invisible stalker, if you have a bag of flower that problem is solved. I don't see why the eidolon wouldn't be invisible if summoned while using the stalkers body. As to limited natural attacks, that may be the case but you could still get other abilities (pounce with an enlarged biped/ monster with weapon attacks for example).
Also marionette possession has a long duration so it'd be quite easy to make your body invisible before starting the shenanigans.
1 round doing nothing while your pal summons a tiger, 1 round casting possession, 1 round summoning eidolon. That's 3 rounds of combat where you could just be killing guys. 3 rounds is a long time just to set up.
The eidolon won't be invisible because "The synthesist wears the eidolon like translucent, living armor. " (from the SRD) Translucent armor, not invisible armor. He may be harder to see but translucent=/=invisible.
So you get extra abilities... that's kind of the whole point of the spell isn't it?
Sure you can make your body invisible. You can also expend more resources to make the rest of your party invisible, or to get the heck out of dodge when they come looking for you.
If you think it's too much don't allow it. I think if your players come up with a tactic like that (and are willing to waste the rounds and resources to pull it off) I'd 'award' them by letting them actually use it.
First round you invisible yourself while friend summons invisible stalker. Second round you possess stalker while he casts buffs on it. Third round eidolon friend casts more buffs. This could be done 100 ft + 10 per level away from where the combat starts.
As to the stalker and invisiblity; the eidolon and summoner are one being. They can not be targeted seperately. If the summon (who is now the stalker) is invisible then so is the eidolon...
Sure looks like a great trick that would be viable rarely and only work for a single encounter. Alternatively: First round summon your eidolon, spend a few rounds buffing then run in and smash face just as easily. Granted, your way would make me feel a lot cooler for dong it.
Not being targeted separately has nothing to do with anything. The stalker is invisible. It is not being targeted with an invisibility spell, it is invisible naturally. The eidolon, by RAW is visible. It does not gain the stalker's invisibility. If the summoner is targeted with an invisibility spell, the eidolon is also targeted. If the summoner is already invisible there is nothing in the rules stating the eidolon becomes invisible when summoned. If the stalker could turn off, then reactivate it's invisibility after the eidolon is summoned that would work, but that's not how natural invisibility works.
If you don't want them to use it just use Lazar's reasoning to limit the spell. I wouldn't because I see nothing OP or exploitative about it, but that's just me.