Prayer

Used Anywhere
Effect Increases hit points or food to 10 on world map, or attempts to remove monster if in combat; does not work in dungeons
Prayer is an great Spell that can really save your life in a tight situation. Used on the overworld, its effect depends on your situation. If there are monsters nearby, it will attempt to remove them. This will get rid of the monsters, but you'll earn no reward. However, if either your HP or Food is below 10, it will increase one of them to 10. (If both are below 10, it will increase HP to 10.) Thus, provided you aren't under attack, you can always survive to make it to town.
Compare Prayer to other Spells
Types: Which can buy Prayer – 4 shown
Name
The number used to select this character type
Number
Each character type gets certain bonuses to their starting statistics
Stat Adjustments
The special ability granted to characters of this type
Special Ability
Fighter 1 Strength +10, Agility +10 -
Cleric 2 Wisdom +10 Spells never fail
Wizard 3 Intelligence +10 Greater spell variety
Thief 4 Agility +10 Twice as proficient at Steal and Unlock
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Prayer

Key a
Used Anywhere
Effect Casts a random spell in dungeons; increases hit points or food to 15, or attempts to remove adjacent monster on world map
Prayer is an amazing Spell that can really save your life in a tight situation. On the overworld, its effect depends on your situation. If there is a Creature nearby, it will attempt to remove that Creature. This will get rid of the monster, but you'll earn no reward. However, if either your HP or Food is below 15, it will increase one of them to 15. (If both are below 15, it will increase HP to 15.) Thus, provided you aren't under attack, you can always survive to make it to town.

In dungeons, Prayer instead Casts a random Spell, including Wizard-exclusive Spells even if you're not a Wizard. Of course, there's no way to predict what will be Cast, so this isn't reliable. There are no negative Spell effects, so if you really need a particular Spell (like Ladder Up, perhaps), you can repeatedly Cast Prayer until you get it.

Prayer

Key 6
Type Cleric
Effect 50% chance to destroy the enemy in front of you
Normal Price 220 gold
Discount Price 136 gold
Bargain Price 84 gold
When the Prayer Spell succeeds, it simply kills whatever Creature is directly in front of you. The Creature's type and HP are irrelevant. As far as I can tell, the success of the Prayer Spell is completely random, and it works half the time. If Wisdom does anything, it may affect the success rate of Prayer, but it doesn't appear to.

As a kill Spell, Prayer isn't bad. It's not as effective as the Wizard attack Spells, but it's much cheaper. It's a good Spell to use if your Stats aren't up to snuff and you just want to get into a dungeon, get some loot, and get out. Balrons are nasty foes and a 50% chance to kill one outright isn't bad at all.