Mystera is the goddess of magic and glyphs. Mystera is well-suited to a variety of races and strategies, but has very few repeatable boons or boons. She also has few offerings to help you in the end-game. Mystera is best worshipped during the early exploration phase when there are still plenty of tiles left to reveal to earn piety, and if found late is usually not worth following.
Mystera is unlocked to the north, in a subdungeon protected by the Guardian of Magic. Although it is theoretically possible to kill this guardian, the intended approach is to use magic to get past him. This can be done by any of the following ways:
1) Using ENDISWAL to dig around him
2) Using WEYTWUT to teleport past him
3) Using PISORF to push him out of the way
Mystera rewards you with piety whenever you cast spells, and deducts piety whenever you kill monsters that deal magic damage. Like playing Taurog, only in reverse, so long as you ensure you cast sufficient spells between each kill you will turn a net profit in piety. Unlike Taurog, the amount of piety Mystera offers is proportional to the mana cost of the spell, so it really doesn't matter which spell you are casting.
Mystera's magic boon is a fairly straightforward boon, but its piety cost increases rapidly and it's seldom worth taking it more than twice. Raising your maximum mana to 15 means each mana potion will restore exactly 6 MP, but beyond that point the piety cost for a mere 1 MP is seldom worth the trade.
One of Mystera's most powerful boons is refreshment, granting you bonus mana regeneration every time you convert a glyph. If you can raise your maximum MP, this can be a major benefit, but it does mean the timing for converting glyphs becomes more difficult to gauge, and races like elves or humans will have to go without their conversion bonus. Races like gnomes, halflings, and goblins have no problems leaving all their converting for the end of the dungeon and they benefit enormously from refreshment. Keep in mind refreshment gets reduced to 25% if you convert to another religion! For obvious reasons, this boon is not very helpful when attempting the Hoarder badge.
Mystic Balance is a double-edged boon that causes expensive glyphs to cost less and inexpensive glyphs to cost more. Wizards, who already have a discount to their spell costs, get little benefit from mystic balance. Berserkers, on the other hand, are largely unaffected by its downsides and benefit enormously from this boon. Ironically, this makes Mystera one of the better deity choices for a Berserker, essentially cancelling out their spellcasting penalty. For most classes, whether you want to use Mystic Balance depends on which glyphs you rely on primarily. Most glyphs already cost 5 MP and are unaffected. The BLUDTUPOWA glyph has no mana cost to begin with and is also unaffected. The BURNDAYRAZ and CYDSTEPP glyphs are the most notable improvements, which makes this boon very useful to fireball spammers and to Warlords. The WEYTWUT glyph is also dramatically improved, and becomes easier to use as a finisher. On the other hand, the PISORF glyph becomes impractical for offensive purposes (it's still functional for breaking through walls and moving monsters, however) and the GETINDARE glyph becomes less mana efficient (though still worthwhile). The BYSSEPS and LEMMISI glyphs are the hardest hit by mystic balance, and are below-average to begin with; they should be consigned for conversion if you select this boon.
Mystera's "Flames" and "Weakening" boons are rarely worth pursuing. Weakening costs an enormous amount of piety, and it's only usable if you don't have any resistances of your own. You're usually better off using a piercing wand or Binlor's stone heart boon, which have no downsides and reduce resistances with less effort. The massive attack penalty on the flames boon makes it a terrible idea for hybrids and even for most spellcaster builds. You will need to be very dedicated to a pure caster approach for either of these to be worthwhile. Most characters will skip out on these two.
For most characters, the best approach with Mystera is to worship early, earn lots of piety, purchase a handful of boons that suit your strategy, then convert out. Unless there are still many glyphs left in the dungeon to leverage refreshment, Mystera doesn't give any short-term boons that can help you overcome a powerful boss. You're usually better off converting into another religion to spend your remaining piety in the end-game. Furthermore, in a dungeon with lots of magic-user monsters you can be facing massive piety penalties for clearing out the weaker monsters. Converting before doing so is highly advisable.
The alternative approach is an extreme spellcaster strategy, fuelled by BLUDTOPOWA. Mystera does not reward BLUDTOPOWA use directly, but as you will be using it to generate mana, which you will then spend on glyphs, the overall piety gain is much higher than for other builds, allowing you to take the more expensive boons, such as Mystic Balance, earlier than would normally be possible and potentially use Mystera to fuel a pact. Depleting your life with BLUDTOPOWA also means you won't be attacking much and have no use for resistances, but can cast a very large number of fireballs, which makes the trade-offs offered by Flames and Weakening much more attractive than they would be for more balanced characters.