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I believe it is instant, which is why you should not play it til the end of phase, where it would heal any hero discarded.
So based on this ruling if we go back to my example...CotH and Jacob are both alive at the end of the battle, both having been healed by Brass Serpent?
They are returned to territory, so the battle is not won by them. You lose the battle, but keep the heroes.
In your original example, neither Jacob nor Captain would be healed. Wait to play the Healing Enhancement until the battle is over.
The reason it wouldn't work in the original example is that Captain is still Negating all Enhancements until Discard phase. If you played it then it would work.
He does? I was under the impression that ongoing abilities on characters like that default to the battle phase only, unless specified otherwise.
So Brass Serpent heals any heroes discarded during the previous phases of the turn. Does I am Healing work the same way? It's worded very similarly.