Notes
Demons are the servants of evil. More or less devoted servants of Zathroth, they cause strife and havoc wherever they appear. Their masters are known as Demonlords, Demon Overlords and Archdemons. Long time ago, Demons used to drop Magic Swords and Coconuts. It is said that in one room behind Hero Cave, a Demon can spawn even if there's people inside, but it has not been confirmed. You can use a Blessed Wooden Stake on a slain Demon to get Demon Dust.
Abilities
Melee (0-520), Huge Fireball (150-250), Great Energy Beam (Life Drain; 300-460) , Energy Hit (Only from close; 210-300), Mana Drain (0-120), Self-Healing, Haste, Fire Field, Distance Paralyze, Summons up to 1 Fire Elemental.
Damage Taken From Elements
Location
Hero Cave, Demon Helmet Quest, The Annihilator Quest room, Ferumbras' Citadel, Goroma, Ghostlands (Warlock area - unreachable), Liberty Bay (hidden underground passage - unreachable), Razzachai, deep in Pits of Inferno (found in every Throneroom except Verminor's), deep Formorgar Mines, during The Demon Oak Quest and in Yalahar.
Strategy
If you don't have some kind of healing, don't even look at them because if you do you are dead. Stand in a diagonal position to avoid the lethal Great Life Drain Beam and, as a blocker, kill the Fire Elementals quickly. You can start blocking them at Level 80 with good Skills with a big waste (blockers with Level lower than 80 will probably die in 80% of the cases). You can hunt them with a team of Mages using Icicle Runes. It's better to use Boots of Haste instead of Steel Boots because Demons are fast and staying diagonal can be hard. Mages can actually block them with full mana using Magic Shield at lvl 80 with Great Mana Potions while someone shoots (not profitable).
Paladins can run them with Boots of Haste, Time Ring and kill them with Assassin Stars or Arbalest with Power Bolts in wide respawns. Using Divine Healing after Demon's Huge Fireball (Radius as high as Ultimate Spells) will keep you out of danger. Just focus in your speed and in your Mana. You can't stairhop them anymore.
Knights can kill them but it will most likely be a huge waste as Demons are strong against Physical Damage.
Mages should use the same strategy as a Paladin, however, using Magma Set and Magic Shield below Level 130 is very recommended. All vocations can use Garlic Necklace to reduce damage from Great Life Drain Beam
Loot
0-310 gp, 0-1 Platinum Coin, 0-6 Fire Mushrooms, Devil Helmet, Double Axe, Golden Sickle, Small Emerald, Fire Axe (semi-rare), Giant Sword (semi-rare), Great Health Potion (semi-rare), 0-3 Great Mana Potion (semi-rare), 0-3 Ultimate Health Potion, Golden Ring (semi-rare), Ice Rapier (semi-rare), Orb (semi-rare), Purple Tome (semi-rare), Stealth Ring (semi-rare), Talon (semi-rare), 0-5 Assassin Stars (rare), Demon Horn (rare), Demon Shield (rare), Might Ring (rare), Platinum Amulet (semi-rare), Ring of Healing (semi-rare), Golden Legs (rare), Mastermind Shield (rare), Demon Trophy (very rare), Demonrage Sword (very rare), Magic Plate Armor (very rare).