I think I was able to collect it almost the instant it was offered. Beware, though, some of the items overlap with Crimson Gorecutter; if you don’t have that yet, Submaripper will set you back (and will make the collectible packs not give you the impossible-to-get purple urchins or whatever they are).
I still need a lot of the Purple Death items to get the Goregutter so for now I’m happy sacrificing that to get the Submaripper.
Probably the one that looks like a purple spiny urchin? If you’re willing to buy the collectible packs (for Odin’s coins) and you don’t need any other items for any other collection, you seem to have something like a 50% or maybe even higher chance of getting one of those (which Purple Death almost never brings) from a collectible pack. Once you need other items for the Crimson Gorecutter collection, that chance becomes much, much lower and you will have to get the other items again (e.g. with Toothless) before collectible packs will give you the otherwise-almost-impossible-to-get urchin-things.
Do you care? Maybe not… Just keep in mind it will push back how quickly you can get Crimson Gorecutter.