Historically, most tombs known to have grave goods in them were robbed soon after the dead person was laid to rest. So why has the tomb that your group of adventurers broken into not already suffered this fate?
Inspired by Ria Hawk's winged mushroom picture:
The winged mushrooms (Lerreta Meia) are a species of giant insect indigenous to the leafy areas of Udnalor, Kingdom of the Gnomes. Outwardly they appear much the same as ordinary giant mushrooms, and they graze in the bulbous fungal glades. However, this is merely an elaborate camouflage: the insect beneath is a fungivore and prone to fly off when it encounters another creature. Sometimes gnome farmers mistake them for crop mushrooms. They're in for a surprise when they do.