What happens after using Spinal Embrace on a Theros God?
Your opponent will gain control of Phenax, then at the end of your turn they will sacrifice it and gain zero life.

When Spinal Embrace resolves, it creates two continuous effects:


You control that Phenax.
Phenax gains haste until end of turn.


and a delayed triggered ability:


  At the beginning of the next end step, sacrifice Phenax. If you do, you gain life equal to its toughness.


So, right after Spinal Embrace resolves, your opponent controls Phenax and it has haste. Then, at the beginning of the next end step, the delayed triggered ability triggers and they sacrifice Phenax. Then they gain life equal to its toughness. However, rule 302.4 says


  Power and toughness are characteristics only creatures have.


and rule 107.2 says


  If anything needs to use a number that can't be determined, either as a result or in a calculation, it uses 0 instead.


When you sacrifice Phenax, it is not a creature, so it doesn't have a toughness. So, they gain zero life.