Arachnis

Arachnis is a rare spider that spawns in Lunarfall and Frostwall during Hallow's End for players who have purchased and used Creepy Crawlers. Among other non-notable items, Arachnis drops the Sack of Spectral Spiders toy. Arachnis can only be looted once daily, but it spawns every five minutes or so.

The Creepy Crawlers usage applies to the garrison, so realm hopping or using Group Finder to enter another player's garrison for this spider will work.

For a brief while, it was possible to look at the loot without triggering completion of the hidden daily tracker. If Arachnis did not have the toy, one could simply wait for the spider to re-spawn and try again. This was changed in an undocumented hotfix, as any items not looted from Arachnis will now be sent through the Postmaster -- which does trigger the tracker.