The Cursed Oasis is an unusual dungeon, composed of two overlapping realms. You will be forced to handle exploration and popcorn monsters in unusual ways. Curses play a large part in this dungeon, so characters that rely on resistances, particularly monks, will struggle.
Monsters: Bandits, Dragon Spawn, Animated Armors, Thralls (Poisonous and Mana burn), and Shades. Monster difficulty is 115%.
Special Rules: You gain one stack of Cursed whenever you gain a level in this dungeon. This even affects Crusaders, which are otherwise immune to curses. When you have any stacks of Cursed, you will find yourself in the cursed realm.
The normal realm is a very large and open dungeon containing relatively few walls, with an oasis in the center that can only be explored via LEMMISI or BLUDTUPOWA. Monsters include Bandits, Thralls, Animated Armors, and Dragon Spawn. In addition, while exploring, you will sometimes encounter lifedrain by an invisible monster. These are actually the Shades in the cursed realm, which you can walk through in the normal realm.
If you have any stacks of Cursed, you will immediately enter the cursed realm. In the cursed realm, most dungeon features on the ground are not available: glyphs, potions, boosters, shops, unused altars, gold piles, subdungeon stairs. Even the dungeon exit vanishes. Active altars, which remain available, are the only exception. Thus, before fighting a Bandit or gaining a level, you should have a plan for killing enough popcorn to clear your curses if you still need to exploit these resources.
The Shades become tangible in the Cursed Realm and provide a valuable source of XP and Curse-clearing. However, because they can blink into blackspace and then lifesteal again, it is risky to fight higher-level Shades before the dungeon is mostly explored.
The Cursed Realm also features a large number of Erosive Eelroots in the locations of shops, boosters, potions, and gold piles. The dungeon is open enough that they don’t usually interfere with getting around; they are useful to build up extra stacks of Cursed so you can avoid getting kicked out of the Cursed Realm before you want to.
Be careful when using IMAWAL, as it behaves in a quirky way in this dungeon. The most important consideration is that if you petrify a monster that exists only in the Curse Realm (like a Shade or Eelroot), the resulting stone wall will also exist in the Normal Realm. This is especially important since the Eelroots are growing specifically on top of noteworthy tiles. So unless you have a way to destroy walls, avoid petrifying those Eelroots that are on top of things you still need to access like stairs, shops or items you still need to pick up. Another interesting thing is that stone statues created in one realm will appear to meld into the wall in the other, making it hard to tell where to break if you want to access what was under the monster.
The primary boss of the Cursed Oasis is the Cursed Dragon, the only monster that appears only in the normal realm. He has decent but unremarkable stats: 546 HP, 86 attack, Retaliate: Fireball, and magical attack. Before the Curse Shade is defeated, he also has Curse Bearer, so you won’t be able to easily fight him before you’ve taken care of the second boss.
The secondary boss of the Cursed Oasis is the Curse Shade, who appears only in the cursed realm. He has only 159 HP and 60 attack, but 60% physical and magic resistances. He also has both 40% Lifesteal and Blink, so you won’t be able to carry much if any blackspace between fighting him and the Cursed Dragon. The Cursed Shade drops the Dragon Soul item when slain, which provides a 15% chance to cast spells for free, and forces you into the normal realm even if you have any remaining Curse stacks. You are not obliged to kill the Curse Shade to win the dungeon.
There are two main approaches to winning the Cursed Oasis, depending on whether you want to kill - or avoid - the Curse Shade.
If you want to kill the Curse Shade, you will need to either bypass its 60% resistances, or simply be able to dish out enough damage to grind through them. Preparing the Really Big Sword helps to avoid most of the Physical Resistance, and it's useful also against the regular Curse Realm shades. However, none of the other monsters have Physical Resistance, and being slowed can be a nuisance; on the one hand, it makes killing low-level Thralls non-trivial due to their status afflictions; on the other, being slowed means you get an extra layer of Curse in melee from Eelroots (and lower-level Bandits), which can also pile up to undesirable levels. Using Binlor Ironshield and reducing the Curse Shade's resistances via the Stone Heart boon can help; consider though that there is only a limited amount of walls available on this level. It's also possible to just keep hitting the Curse Shade until it dies, as its stats are not too high. In any case, be mindful of the Blink.
If you want to avoid the Curse Shade, you will want to have a reliable way to clear - or avoid - Curse Stacks. The obvious way is to play a Crusader, who is immune to monster-inflicted Curse stacks. If you would rather play another class, you may still do this by clearing the curse stacks inflicted by the Curse Dragon in-between strikes. While you can always just leave a couple of low-level monsters and kill one each in-between strikes, a potentially even better way is to use The Earthmother's Greenblood boon to this effect. And lastly, you may also experiment with chaining PISORF against the Cursed Dragon, if for nothing else than to have an excuse to play a Binlor-worshipping Orc Bloodmage.
Classes recommended for this level will partially depend on the strategy, but on top of the above mentioned Crusader, the Berserker is also a good pick, as his Magic Resistance gives a natural edge against the Animated Armor and the Dragonspawn, as well as the Cursed Dragon boss itself. Just make sure you fight these creatures while you are not Cursed.