SPOILER follows:
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The poison was never used on the Spirit. It was used on Relmar's axe blade as he poisoned the tree, the one that is the source of the Spirit's power, by hacking into it. That's what made the Spirit and all the creatures under its sway go mad. After the deed was done, a few of Relmar's cohorts escaped back to the world througha portal they cast by magic, probably taking what was left of the poison back with them to their original campsite. At least this is what we are supposed tyo be able to piece together after reading the Spirit's thoughts after the cure is given.
Therefore, the poison itself has no purpose in the quest other than evidence.