The event is ending soon. I have used 15 event gemstones at the fortress and was only able to get 2 of the brilliant young acromantula. I’m getting desperate is there anyway to guarantee I get the last one or am I going to be s.o.l. ?
I haven't tested the theory, but considering both the brilliant Runestones and registry stick around after the event; Logic would dictate that you can still use the Runestones to get the Spider at any point.
If you're out of Runestones, then I think you're out of luck until the Event rolls around again (If it does)