There is a rare chance that a lesser demon will drop a rune med helmet, but most of the time they drop ashes, fire runes or little amounts of gp.
i know for sure they drop rune med helms and fire runes
read the link and scroll down to 'drops'
well, the only thing black demons drop is around 200gp and lv 3 clue scrolls. Greater demons dont really drop much. Lesser demons drop rune helm sumtimes (i no this as a fact cause it happened to me)
lesser demons drop black 2h sword and sometimes mithril items.
The Demons that are known to drop Dragon Platebody Parts.
Their most sought after item is Rune Medium Helmet, that is the best drop they offer.they also drop serragetoned ashes and there is greater lesser demons that drop sometimes5% of dropping corrupted dragon.
Greater demons drop Rune Full Helms and Rune KitesLesser demons drop Rune Med Helms (Rune Helms)Cockroaches drop Rune Square Shields and Scimitars
Kill lots of lessers demons and greater demons, they drop rune armor.
They are said to drop rune full helmets but the chances of getting one is unlikely
Black demons drop a wide variety of items, including coins, runes, herbs and charms. However, they do not drop clue scrolls. Some of their more common items include steel battleaxes, black hatchets, swords & 2 handers, uncut sapphires, adamantite bars, defence potions and lobsters. On rare occasions, they also can drop tooth half of keys, rune armour, dragon med helms and starved ancient effigies. This is by no means a complete list, however it should give you a flavour of what possible items await.
lvl 3 clues are dropped by a multitude of monsters, but ones that ive gotten them from include hellhounds, black demons, greater demons, and metal dragons (which also drop elite clue scrolls, i think. At least Mithril ones do.)
They will attack you, and they can drop loot. see this page: http://www.tip.it/runescape/index.php?rs2monster_id=238
If you mean in RuneScape, then they are in the Abyss, and the Canifis slayer tower.
tormented demons, after the "while guthix sleeps" quest