The angel's chance encounter with the succubus happened when the latter went to the edge of Heaven and tried to seduce him. Ellie was planning to increase her rank in Hell if she succeeds. Tali fell for Ellie's seduction, however, the two did not expect that they would fall in love. This resulted in Ellie getting pregnant, and this conjugation between a demon and an angel turned out to be highly forbidden in both Heaven and Hell.
To escape prosecution, Tali and Ellie came to Earth and asked John Constantine for help. Constantine agreed to help them, and he took the couple to a flat which was covered with powerful magic to hide its prescence. However, the magic failed when Ellie started to give birth and was soon discovered by a league of the highest-ranking angels in Heaven. They then killed Tali and took the baby.