IMAWAL ("I'm a Wall") is one of the Glyphs you can find in a game of Desktop Dungeons. When used, this Glyph will turn a monster into a wall tile. This Glyph may spawn randomly in any dungeon and does not need to be unlocked.
IMAWAL is used by clicking it, and then clicking on the target monster in the dungeon. That monster will be the target of the IMAWAL effect.
When used on a monster, IMAWAL will turn that monster into a wall tile. In other words, the monster disappears, and a wall tile appears in its place. This wall tile is no different than any other wall in the game.
Turning a monster to stone is not considered "defeating" it - it will grant you no experience points. However, for all intents and purposes the monster is "gone" from the dungeon. It's worth noting that using IMAWAL on an enemy grants more points at the end of the game than killing it does.
IMAWAL has no effect on Boss monsters. If you try to cast it on a boss, you will fail, losing no Mana and having no effect. However, read more below about Mystera Annur for a possibly unintentional (and very powerful) exploit that stems from this.
The intended use of the IMAWAL effect is to allow you to remove a high-level or otherwise dangerous monster by turning it into a wall, and then destroying it with the ENDISWAL Glyph or any other similar effect. Unfortunately, if you already have ENDISWAL, this point is almost entirely moot (you can carve a path around the monster with just ENDISWAL).
IMAWAL is unlocked from the moment you install Desktop Dungeons. It has a certain chance to appear in each dungeon you play, supposedly the same chance as any other unlocked Glyph (though this has yet to be proven conclusively).
Joining The Earthmother's religion will automatically spawn an IMAWAL Glyph on the same tile as the Altar, assuming the Glyph does not already exist in the dungeon (and has not yet been picked up and converted by the hero either). If created in this way, the Glyph must be picked up before any further worship of Earthmother can take place.
As mentioned above, it is possible to use IMAWAL and ENDISWAL together to remove a monster from the maze. You cast IMAWAL on it, and then destroy the resulting wall with ENDISWAL. This grants you no experience points whatsoever. Also, since the only reason to remove a monster instead of just kill it (even if you can't kill it) is to reach something that's behind it. This can, in any circumstance, be accomplished with just a casting of ENDISWAL, rendering IMAWAL completely useless in this regard.
IMAWAL and the Gods
Several Gods in the game have various opinions about the use of IMAWAL - they may favour it or reject it, depending on their own particular creeds.
As the Goddess of Nature, The Earthmother appreciates actions that preserve it, among them being the use of IMAWAL to create more earth. When joining her religion, Earthmother will attempt to generate an IMAWAL Glyph on her altar tile. This will succeed if the Glyph hasn't been spawned elsewhere in the dungeon. If the hero has already found IMAWAL and/or converted it, the Glyph will not be recreated.
Each successful use of IMAWAL on a monster will grant you +10 Piety with The Earthmother. This is one of only two methods to gain Piety from her.
The other method is to use one of Earthmother's Boons, "Plantation", which grants +10 Piety for each visible blood-stain in the dungeon, and turns all those blood-stains into Plants. Note that Plants are valid targets for IMAWAL, and you will get +3 Piety from The Earthmother for each plant turned into a wall.
A common trick is as follows: Kill 10 monsters. Use IMAWAL on two other monsters to gain 20 Piety points. Then use those 20 Piety points to acquire The Earthmother's "Plantation" boon. The blood-stains from those 10 slain monsters will turn into plants, and you'll gain +100 Piety. This Piety may then be used to purchase The Earthmother's highest boon, "Stoneform".
Unfortunately, even this trick comes with various problems. First of all, by the time you've killed 10 monsters, you'll have already gained several levels and so the boon itself (granting extra stats per level-up) isn't as useful. In addition, you've now got a dungeon full of plants, which block movement just like regular monsters and can't possess 100% resistance to physical attacks. You can easily kill these plants with BURNDAYRAZ, but each plant killed will cause a -15 Piety drop from The Earthmother, which can easily lead to losing your Health Regeneration.
In other words, unless the dungeon plays specifically into this trick, you are unlikely to gain much or anything from it.
Surprisingly, due to what is possibly a programming error (or an intentional caveat), Mystera Annur is the primary reason to find and keep IMAWAL. If your character is a spellcaster, particularly a strong one that relies on BURNDAYRAZ, finding Mystera and IMAWAL can easily win you the game. The reason lies in the combination between woshipping Mystera Annur and using IMAWAL.
Mystera grants +1 Piety point per use of any Glyph. This of course includes IMAWAL. Normally, IMAWAL is not a good way to gain this Piety, as it costs a good amount of Mana, and creates walls that may block your travels.
As mentioned above, casting IMAWAL on a boss will fail - costing you no Mana, and having no effect on the boss. However, it DOES count as a single use of a Glyph to Mystera Annur, who will award you +1 Piety for this failed attempt!
The resulting exploit should be obvious: If you find IMAWAL and the Altar of Mystera Annur, take the IMAWAL Glyph and go look for the dungeon Boss. Repeatedly cast IMAWAL on the boss, having no effect and costing no Mana, but gaining you +1 Piety point for every use! Once you reach 20 Piety, acquire Mystera's "Faith" boon to double the amount of Piety you get from each use of the Glyph (this just helps speed things up a little). Continue casting IMAWAL on the same boss again and again, buying all of Mystera's boons as soon as you can afford them. Finally, get some extra Piety (around 50 will do) just to be on the safe side, and convert IMAWAL (you don't need it anymore).
At this point, having all of Mystera's boons, you have become an extremely powerful spellcaster: Your Maximum Mana has gone up by 15, your BURNDAYRAZ is 25% more powerful, and all monsters in the dungeon have lost all their Magic Resistance (if they had any), including such powerful bosses as Iron Man or Gharbad the Whoa.
From this point on, you can simply use BURNDAYRAZ to defeat all your enemies.
As with any other Glyph, Taurog will penalize you -5 Piety points every time you use IMAWAL, whether or not it was successfully cast.