http://cloud.steampowered.com/ugc/576683685656955300/7A87D6F636DAC3CC647...
In the middle of the fight, Vanaduke just went "poof". Completely disappeared.
The battle music's still going, and the gate won't open.
We were in his second phase.
Needless to say, this is disappointing, as we were doing quite well, and could have beaten him easily. Freeze bombs for the win. :)
Its been proven that it just happens randomly when hes frozen at a moment when he goes invulnerable for a phase change, or while trying to execute a dash. Some suspect he falls through the map, but it seems more likely related to breaking his animation sequences. In any case, it occurs extremely rarely, and you can get GM help by clicking the gear, support, and opening a support ticket (NOT the F2 bug report).
The typical GM solution is to spawn a new clockwork instance, invite you into it, and start you at the clock work terminal. They'll wave the elevator fees, activate the elevator at each depth for the token counter, and drop you off at the Throne Room. You'll have to fight him all over again, but at least it'll reset the Ressing cost and they usually give you capsules and vials to help with the fight.
This has been happening for a very long time; it hasn't been entirely fixed because they aren't able to re-emulate the event, or at least reliably. Something like that was stated by a GM if I recall correctly.