You might be asking, "where is Duskfall Island?" Maybe you accepted the quest, left the area, and later forgot where you needed to be to do the job. The elixir will respond with "You need to be in Duskfall Island" if you're not in the right zone for the quest.
The quick answer: go to the quest giver, Seer Malune (Talador 65.4, 50.7). It's the island southeast of Tuurem, the one with the Terrokar Refuge flight path. Don't try the elixir to the southeast of that FP, though; go to the quest giver's temple first.
I had a bit of trouble finding Duskfall Island, with no help from the in-game map, and it isn't in the wowhead database. At first I tried southeast of Terrokar Refuge, the FP with the bird people. This area is cut off, zone-wise, from the rest of the island, so that the elixir wears off as you try to go northwest through the Terrokar Refuge area; and you will not make it to the objective location, which is Aruumel (Talador 68.8, 55.2).
Use the elixir in the temple where the questgiver stands. If you use the elixir anywhere else on the island you may be attacked immediately by a gang of patrolling voidwalkers, who can be a nuisance at equal level. Alternatively, you could go to the objective location and use the elixir there -- although I've only done the quest once so far and I'm not sure if the voidwalkers patrol through Aruumel's clearing. You might as well do the elixir at the temple and clear the path southwest to the clearing, then run back to the questgiver the same way.
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