Hey there. Necrochasm's catalyst is unique in that you can work on the 'to insert' requirement before even earning the catalyst. This count, of Oversouls collected, will be hidden at first, if you do not own the catalyst, but it is counting in the background. The catalyst itself needs to be earned from two timed plate puzzles within the raid itself. This is the only place to earn it. If you do this, the triumph and counter will show, at whatever your current count is. However it is possible to complete the requirement before earning the catalyst, as there is an RNG chance of it auto completing. In this case the triumph will show up and be complete, but this in itself does not give you the catalyst. It just means that when you DO obtain it, from the only source, mentioned above, you can insert it immediately without needing to do anything else.