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 is deactivated if you convert to another religion!
Mystic Balance is a double-edged boon that causes expensive glyphs to cost less and inexpensive glyphs to cost more. Characters using HALPMEH or PISORF will want to avoid this boon, while those using APHEELSIK, WEYTWUT, BURNDAYRAZ, ENDISWAL, or CYDSTEPP will get a significant benefit. Wizards, who already have a discount to their spell costs, get little benefit from mystic balance.
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 much better off using a piercing wand or Binlor's stone heart boon, which have no downsides and reduce resistances much faster and with less effort. The massive attack penalty on the flames boon is so enormous that only the most dedicated fireball specialists should ever consider it.
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.