Hmm... I just wondered why the PC should not wait with the search of the Daemonweapon till -AFTER- the completition of their primary goals. My answer to this is...
Changes: Navius Merth & the Liberatus Daemoni
While the PC arrive at the Weeping Halls another group of Heretics is already at large. An Apostate of Chaos Undivided going by the name of Navius Merth. He is the leader of a group he dubbed “the Liberatus Daemoni” and is on a crusade of his own: to release entities of the warp from daemon weapons and other artefacts.
The PC will notice Navius preaching in the Weeping halls, trying to recruit followers for his actual cause. He is intend to find a Daemon Weapon which is hidden in the Vaults to release the spirit within and to thereby gain the favour of Chaos. He already has small number of followers with him. If PC try to attack him directly, half of his followers will flee while the other half will attack. Navius in tunr willl grab an amulet around his neck and begin to cry out in a language unknown to the pc… with the result of a Netherswarm (see Daemon stats in the core rules) pouring into the surrounding reality, fighting and killing for 5 rounds will blocking all sight. Afterwards, the Netherswarm disappears and Navius has escaped.
Otherwise, Navius will have departed after the Heretics return from the wreck. All in all, Navius is no real competition. But if they intend to search the Daemon Weapon after they have dealt with their Rivals will arrive only to see Navius finishing a ritual and thereby destroying the weapon. The Heretics could of course try to make use of Navius and his groups, joining forces with him. If they want to use him as pawn in their own plans, they will need to path a difficult(-10) Deceive test or otherwise Navius will know what the PC are up to.