Caladbolg

The sword with a dull color had a triangular shape that became narrow as it reached the peak and there was one more handle you could grab on next to the hilt just like an oar of a boat.

It is called the sword of thunder used by CuChulainn's enemy Fergus mac Roich.

Caladbolg wasn’t a normal weapon. It was one of the strongest magic weapons in Erin that was comparable to Gae bolg or Claiomh Solais, the sword of Nuada.

There was a time in the past that Fergus mac Roich, the enemy of Cuchulainn, destroyed the peak of a mountain with an attack of Caladbolg. But Caladbolg required an incredible amount of magic power to execute that one blow. Because of that not anyone could use it. If someone that lacked the abilities used it, then they may get their magic power, stamina and life power sucked dry by it and die as a result.