Euderus set, or the "crypt keeper wasp" is a small Eulophid parasitoid of the gall wasp Bassettia pallida.
The female Euderus set will search for the galls or "crypts" induced by the gall wasp Bassettia pallida, but possibly also Andricus quercuspetiolicola and others. The female E. set will then oviposit in the chamber of the gall. When the E. set larva hatches, it will burrow into the host wasp. It then manipulates the host to speed up its development, metamorphoses into an adult, and chews its way up to the surface months earlier than normal. When the host wasp does this, the burrow isn't wide enough for it to emerge from the stem, and its head becomes stuck. The parasitoid then consumes the host and chews through the host's head to emerge as an adult. The larva of E. set overwinters in the gall, eating the host, and emerges the following spring. The mechanism used to manipulate the host is unknown.
This female apparently got overly enthusiastic and took things one step further by wearing her host's head as a costume. Happy Halloween!