The sacrifice ritual, revisited

In the US version, the Sacrifice Ritual was pretty simple. It let the caster automatically succeed on the Ka roll to cast a spell after the initial occult technique roll. To do this, the caster bled on a physical manifestation of the spell's element and spent a permanent point of Ka. This wasn't allowed for Moon spells because it was part of the ritual to become Selenim, but no rules were provided for what would happen if a PC tried anyway.

Notably, the sacrifice had to be announced by the player before the technique roll. This meant that the Ka could be wasted. Since it was permanent Ka, it could only be regained through experience.

The sacrifice ritual couldn't be made for Summoning or Alchemy.

In the French version, things were a bit different. In 1e/2e rulebook, it was simply stated that Nephilim of the Moon couldn't attempt this because it was part of the ritual to become Selenim. It wasn't clear whether this meant all Nephilim couldn't sacrifice for Moon spells, Onirim couldn't sacrifice for any spells, or a combination thereof. 

In the 2e supplement Le Ka, it was specified that this instance of the Sacrifice Ritual increases Black Moon POT by 1 each time. It's still not clear whether this applies to Moon spells, Onirim only, or a combination thereof.

In 3e, sacrifice could be done for spell's of any element. Spells were only cast with a single roll rather than two, so players didn't have to worry about wasting Ka. No material representation of the element was necessary, just shed blood on the ground and the roll automatically succeeds. Onirim were forbidden from doing it because initiates transformation into a Selenim, but no rules were provided for the consequences even though Selenim were in the rulebook and there were rules for Black Moon contamination!

In 5e, sacrifice no longer involved shedding blood either. The caster simply spent 5 "Sapience Points" and the roll was automatically successful, but this couldn't be used for spells the PC couldn't ordinarily cast. PCs in this edition accumulated SP (Occult Development Points in the US version) like experience points to spend on developing their magical techniques.

The latest revision to the US version's rules comes from the Enlightened Magic supplement. In these rules, POW (Ka) rolls are not made for magic. So you don't have to worry about wasting POW on a failed roll. No option is presented for sacrificing POW to automatically succeed, and shedding blood is never mentioned. POW is only spent to enchant certain permanent effects, like enchanting a spell into an object or enlightening another person. I personally prefer this revision and would use it as the default in my campaigns.