In her current incarnation, Mystera has returned to be a god for pure fire-ballers. Mana and Mana Leak both cripple your ability to deal physical type damage (including PISORF), while flames cripples your regular attack. The result is a character whose only relevant means of dealing damage is BURNDAYRAZ. Her weakening boon is only useful for these kinds of characters, and is basically an inferior and overpriced version of Binlor's Stone Heart. For characters who are not specialized with the BURNDAYRAZ glyph, her only remotely usable boon is refreshment. Since this effect does not carry over on conversion, she is largely useless to anyone but a pure BURNDAYRAZ caster.
While this new Mystera is certainly strong, she is ridiculously overspecialized to the point at which only Assassins and Wizards need apply to her religion as anything other than a piety farm. A long-standing issue is that she is ridiculously dependent on BLUDTUPOWA both to gain piety and to perform effectively. I respect her double-edged nature, but she needs to be applicable to a wider range of characters and approaches rather than a BURNDAYRAZ/BLUDTUPOWA combo.
As always, I have crazy suggestions of my own:
Mana Leak: All spells cost -1 MP to cast, but you take 1 point of damage per character level when you cast them. This would complicate a BLUDTUPOWA based regen game, and it would keep the low-cost spells (PISORF and HALPMEH in particular) under control.
Magic: Gain +1 max mana, but you are exhausted when below 15% health. Every time you select this boon, the exhaustion threshold rises by 15% (maximum 100%).
Flames: -50% attack bonus, fireballs deal +1 damage per level, gain a x3 burning attack. This means you deal 3 points of damage for each point of burning on the target (still only add one point of burning per attack).
Weakening: the BURNDAYRAZ glyph lowers enemies physical resists by 10% per hit but lowers your physical resists by 1%, your physical attack lowers enemies magic resist by 10% per hit but lowers your magic resist by 1%.