I'm currently playing Redguard and ATM I'm with the quest to retrieve Hayle's soul...

They said that Hayle was born under the sign of the serpent...
But how can someone be born under the sign of the serpent if the serpent hasn't his own month?

The serpent moves through the sky and "attacks" a different constellation each time. The month of the besieged constellation becomes the month of the serpent.

Have you also heard of the Shadowscales? They are Argonians born under the sign of the Serpent who are brought into the Dark Brotherhood.

Also, in Morrowind and Oblivion, you can choose the Serpent as your birthsign.

It's possible to be born under it and it's as proweler has said.

^ Wrong sign, and it's irrelevant pap anyway.

Teinaava in oblivion says argonians born under the sign of the Shadow become shadowscales.

Say this reminds me of a question i wanted to ask and forgot, if you play as an argonian with the serpent sign in oblivion do you automatically get recruited into DB or does dialoge with teinaava and the other one change?

Shadow, not serpent, allows for argonians to become Shadowscales

My gut instinct says no. But I know that the DB questline had some of the most in-depth dialogue in the game so I don't know. Maybe someone who actually has access to the editor could help me out *nudge*.

I belive the dialouge changes slightly if you're an Argonian, but I don't believe it changes any further if you take the Shadow brithsign.