The Demons' Reagents

Just as we require reagents to keep this portal closed, the cultists must require reagents to keep it open. If we were to destroy those items the portal would close forever. There's only one problem. We might eventually need to use the portal for our own purposes. If it closes we could lose a powerful tool.

If we take the cultists' reagents, then we could prevent them from strengthening the portal. That would make our job much easier. We could also store them for later use so that we could suddenly open the portal when the time suited us.

I'm pretty sure the reagents the cultists use are imbued with Astral energy. Hunt them and steal anything that fits the bill. Bring the items back to me and I'll make sure all the Imperial Sentinels know of your deeds.