The creature clung to the wall. It's powdery body leaving an a white trail as it moved toward the party.
"What kind of spider is that?" Nimble asked.
"It looks like a crab spider, but I've never seen one that looked like that before." Feris answered.
Rathgar drew his throwing hammer, and tossed it at the strange creature. The hammer smashed into the creatures thorax, causing it to break apart into a small cloud of white dust which settled slowly to the ground around the hammer. "There, now we don't have to worry about it." He started to move to retrieve his hammer when Feris stopped him.
"Lets give that a minute to settle. We probably don't want to breath any of that in" Feris cautioned "... but I do want to get a sample."
Armor Class: 9
Hit Dice: 1/4-1 (T-M)
Move: 30' (10')
No. Appearing: 1d4 (2d4)
Save As: F1
Treasure Type: Nil
XP Value: 5-10
Type: Lowlife (Uncommon)
Arachnimold are actually a form of parasitic fungus that colonizes on spiders, killing them, and taking over their bodies. Primarily this is to increase the distance they can spread themselves. They will attack spiders on sight, and will react violently when threatened. They will constantly shed spores, leaving a faint white trail wherever they go. When they are killed they explode in a harmless (to non-spiders) puff of spores.
Picture and inspiration via Pumpkin Rot