They will attack you, and they can drop loot. see this page: http://www.tip.it/runescape/index.php?rs2monster_id=238
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.
lesser and greater demons
The Demons that are known to drop Dragon Platebody Parts.
Greater demons drop Rune Full Helms and Rune KitesLesser demons drop Rune Med Helms (Rune Helms)Cockroaches drop Rune Square Shields and Scimitars
in the wilderness level 56
Greater Demons, Lesser Demons, Dragons ( Green or blue ), Fire Giants, cockroaches.
If you mean in RuneScape, then they are in the Abyss, and the Canifis slayer tower.
These nonmember monsters drop adamant items: White Knights Moss Giants Ice Giants Ice Warriors Green Dragons Lesser Demons Ankous Cockroach Soldiers Greater Demons Also some revenants drop adamant items.
82. there are many types of demons such as greater lezzer demonds:92 lesser demons:82 83 84 black demons :138.soo hope i helped
They are said to drop rune full helmets but the chances of getting one is unlikely
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.)
Kill lots of lessers demons and greater demons, they drop rune armor.