Any suggestion? So far I have received two visits and now I must return to the temple and decide what to do, it seems a choice between betraying or geting a very powerful enemy
I don't feel like there is a 'correct' answer there (although do make sure, when you are talking to the Nameless, that you say the answer you actually want to say, haha)
The Nameless clearly does not have the best of intentions for, well, anybody. You are likely included in 'anybody'. Working with him may result in big perks, but it also could cost you perks with other people.
Then again, he may be too strong to properly stop all the same. Appeasing him might mitigate what he does.
I personally went with standing firmly against him - I see my herbalist as the only entity standing between the Nameless and Bareshade/Lizards/Orcs/Northcrest, haha, so I'm more than willing to accept the Witch's help. (Plus I mean, I /said/ I would help out the witches in advance).
If the goal is just sexytimes, then you'll probably get that either way, just to varying degrees haha.