The encounter against Siegecrafter Blackfuse is a single-phase fight. Your raid will face not only Blackfuse himself, but also several types of adds.
There are two conveyor belts, one located at each end of the room where you fight Blackfuse. At regular intervals throughout the fight, several inactive adds will be brought it on the north-western conveyor belt. Your raid members can move up on the conveyor belt and attack these adds, but only one can be killed, as the remaining ones become immune when one dies. Soon after that, any adds that were not killed will appear on the north-eastern conveyor belt, where they cannot be attack, and from where they use their abilities against your raid for a certain amount of time.
We have released a video preview of this encounter, based on our testing on the PTR. While brief, this video can give you a good idea of what to expect from the fight. We will release a full-length video guide as soon as we have done the fight on live servers.