Mana Transfer: This is a sorcery which allows a mage to transfer mana from himself to another mage by participating in a ritual, keeping our strongest mages full with mana when necessary.
This isn't extremely powerful on its own, but I'd be interested in stealing mana later.
Currently, your magic-users are able to work together in rituals to combine their available Mana reserves, allowing them to cast high-Mana spells - including spells they could otherwise not perform on their own. This process necessitates that the Mages involved be willingly participating in the ritual. This design is a bit redundant, since you can already achieve the same basic effect with what you currently have.
As mentioned in the "Magic" section of the rules:
Because of the integral nature of Understanding in spell-casting, certain aspects of nature can not be directly ensorcelled as they can not be properly Known; the Soul, the Mind, and Magic itself, to be name a few.
Stealing Mana would require an understanding of magic we simply do not possess at our current level, which would put it firmly in the "Impossible" difficulty catagory. How and where Mana is "stored" and "accessed" isn't currently understood, nor is where it comes from or what it consists of.
Balance-wise, stealing Mana from enemy Mages is dangerously close to Anti-Magic, A.K.A. the "No Fun Allowed" spell. Rendering a side's capability to use their fundamental resource inert is not something I can allow, and especially not before the game even starts.
Sorry guys, gonna have to say no on this one.