|During the 3rd Era and the Oblivion Crisis, the Necromancer cult led by Mannimarco undertook a full scale attack on the Mage's Guild in Cyrodiil, almost unhinging it. In the process, the Champion of Cyrodiil recovered the Bloodworm Helm and it was entrusted and secured by Arch Mage Traven. Upon his sudden death at the hands of the necromancers the helm went missing, thought to have been reclaimed by the Worm King, but even after his death, the helm never resurfaced.
It is rumored that a void in leadership amongst the necromancers of the Order of the Rose after Mannimarco's death caused the helm to change hands many times. Eventually the necromantic cult was rooted out almost entirely and Cyrodiil was purged of their taint. I did find that prominent members of the order moved northward into Skyrim and High Rock, so it is likely that the helm is still held by a prominent member of the cult.