The games are inconsistent on the Ritual, giving different powers. This makes the Ritual birthsign difficult to implement. I have some ideas as to how to implement it, but the opinions of Elder Scrolls fans are paramount.

Morrowind/Oblivion

Lesser Power: Blessed Word - Turn Undead

Greater Power: Mara's Gift - Restore Health on Self

Skyrim Greater Power: Animate Dead

One original thought I had was to tie the powers to Anu and Padomay (more commonly known as Sithis) the twin creator deities. Anu is tied to order, so turn undead makes more sense, while Padomay is tied to chaos, so animate dead fits.

The other thought I had was to make many potential Ritual powers, where those born under the Ritual are granted a specific boon by a specific deity depending on the circumstances of the character's birth (represented by the choice of the player and GM). This makes sense given the name of the Greater Power granted by the Ritual in Morrowind and Oblivion, "Mara's Gift."

Please share your thoughts on this.

EDIT: Plus there's this quote from UESP under Lore:Astronomy