First of all thanks for the help.
But let me make things a bit more clear.
I've found Vendrick (the secret boss fight)(and defeated him) cause a friend accidentally told me what to do.it.
I've just started ng++ and that NPC you are talking about is probably the old man on the weelchair.
I've found him two times:
1) At shaded woods.
2) Beneath the Mordor. (I don't remember what the actual name of the castle is)

I suppose I have to find him once more...
Good, I will find him eventually.I already have some places in mind.

After finding him for the 3rd time, I suppose it's not that difficult to find the Dark Lurker and the Covenant.
Anyway...I'll try.
Thanks.

God luck Finding him, by the way the name of the castle is drangleic

He's on the area where you fight that Butcher boss on the Gulch. Remember two Giants that gave you a secret Key? You need to use it to find him a third time, he'll allow you to join the covenant. From here, you can fight Dark Lurker.

Well. if you want to find him by you own. it is kinda difficult.
I searched the internet when I played DS2.
you have to find all 3 old man in some order I remeber.

And light all 3 altars. In the last one, after you jump down(you have to wear a ring, which i dont rember what is the name) into abyss, you will him.

You don't need to wear any ring ,you are thinking of the abyss in dark souls 1.

Darklurker (like most of the bosses in the disappointing DkS2) is only actually difficult if you aren't a caster (which 99% of the community is, given how unbalanced the builds are this time) And he's only difficult because of FromSoft's cheap design to just have the players fight 2 of them at the same time, rather than making his AI and move-set challenging

I don't know how to double quote

@Kane1389 

Edit: Nvm, my memory failed me; it was actually the opposite of physical.