This was originally posted on my Google+ page
In my own roguelike game, Voyage to Farland, I've tried to add more depth to the gameplay to distinguish it from a dungeon crawler & other games that rely on grinding. One example is the Vial item. From the in-game description of it, a Vial is "a cursed vial that when thrown, can capture a monster's soul!"
But you should always remember that in Voyage to Farland, your hero character is just another monster, albeit a cute one whose movements you can control. So normally, you'd throw a Vial at a monster to "capture its soul" or essence. Later, if you drink the essence, you'll turn into the monster until you move to the next dungeon floor or explicitly revert to human form.
There could be several reasons you might want to change into one of the game's monsters -- some monsters have pretty powerful ranged attacks that can be useful, e.g. the Nosferatu's "Evil Eye" attack, the ViperBeetle's "Spin & Toss" attack or the SparkDroid's "Lightning" attack. When you become a monster, you can also take advantage of its inate abilities -- a Ghost can pass through walls freely, and the GrayLady can move at double speed compared to other monsters.
This mechanic in the game is borrowed from Mystery Dungeon: Shiren the Wanderer's "monster meat" gameplay mechanic and it's interesting enough to merit a dedicated dungeon for learning how to use the item wisely. The specialty dungeon is called The Vial Trial and you can get to it by clearing certain events on the Thatched Hamlet floor.
But the ability to turn into a monster isn't enough in designing a complex roguelike game. For me, item orthogonality is very important -- all "good" items should have some potential disadvantages, and all "bad" items should have some beneficial uses occasionally. It keeps the game interesting and adds yet another layer of replayability to the already compelling gameplay that procedural environment generation brings to a video game.
A Vial is great for throwing, morphing into a monster and then throwing down. Yet there are monsters in the game who delight in picking up stray items and tossing them at the hero. The Catapult family (felines on mobile mini-catapults!?) does just that. If it's standing on a sword, it'll lob it at the hero doing damage. If it's standing on a medicinal herb, it will toss the herb at you, which you'll feel compelled to ingest, maybe saving you from death in a tricky situation. And here's where the subtle twist in item design comes into play -- if the Catapult runs across a vial, it will toss the Vial at the hero. Perhaps the Vial contains the essence of a Wraith (level 3 Ghost). If so, you'll morph into a Wraith and you can pass into a nearby wall and attack any monsters that come near you. Their attacks can't affect you, but yours will do high damage to them! Cool, huh?
But if the Vial was empty when tossed, that's another story... Your essence will be trapped for a few turns and any nearby monsters can approach and pound on you. Just pray that your level was high enough to withstand the onslaught, or it may be YASD (Yet Another Stupid Death) and back to the starting village.
From the Android version of Voyage to Farland
One of the Dwarf Fortress taglines is "Losing is fun!" and most roguelike gurus appreciate that fact. It's part of the draw of these games. They're not for the faint of heart, but for those of us willing to learn from our mistakes and grow from them, roguelike games are extremely rewarding.
So in summary, "Use the Vial, Luke. Use the Vial." Just don't leave one lying around near a Catapult when you're playing Voyage to Farland.