yeah, the artifacts that enable abaddon to shut down the power on the fortress
move within 30cm is tempting but I was wondering if that's possibly a bit easy. Hence my dice rolls or a boarding action. Much slimmer chances than the fluff would indicate (where abaddon had an easy time of it) but I want to give the blackstone a good innings.
See, I'm playing in a campaign. If the sub-sector's blackstone is present in a battle where chaos is attacking, then the chaos player gets the opportunity to nab it. Until he does so, he can't use a blackstone in an Exterminatus! scenario. (Other blackstones are in engaged elsewhere)
If he does succeed then the blackstone can no longer be used by imperium player.