This is an extremely easy quest for level 25ish. Can be done earlier, but the Witchlamps can be tough as they are casters (necros?) and will DoT (Boil Blood, Darkness, etc.). They are not social and will not add, or aggro for that matter.
They are more plentiful at night and it would seem that the more you kill, the faster and more powerful they spawn.
Their favorite spawn area is between the Signpost and the Dark Tower to the north of zoneline.
They drop the NO TRADE Witchlamp Lightstone item (resembles a bloodstone gem) each kill...confirmed. (I've done this quest at least a dozen times.) The NO TRADE Witchlamp Lightstone items are pre-lootable, just get Task before hand in.
I've done this Task on my necro, my cleric, my BST, and my wizard...however, my Warrior was unable to get the Task. When hailed, she said (to Warrior lvl 26.):
"Gaa!, don't startle me.! If I mix these these magical ingredients incorrectly I could tear open a rift to another plane of existence..."
I was boxing my wizard (same level, 26) and was able to get the Task for him.
Perhaps non-castes/priests cannot get the Task...? (I realize melee classes do not need Clarity pots, but was doing it for extra xp.)
XP for this Task is about 2 blue at lvl 26.