I read about how overpowered the Pankratosword technique, but there is hardly any information about it and the Redguards don't use it anymore. Is there any official sources regarding this technique and whether it is directly responsible for most of Yokuda being destroyed?

I don't play ESO, but I am aware that at least one in-game character- http://www.uesp.net/wiki/Online:The_Warrior_(Celestial)- uses a Shehai. He also has quite an interesting line:

The Shehai of a first rank Ansei sank Yokuda. This Warrior's Shehai is beyond first rank.

If there are more Shehai-users in ESO, I don't know them.