I want to create a sword that can pass through an opponent's weapon/armor (mostly made of metal like steel/iron, bronze, copper, lead, silver, gold, etc) like it was ethereal or hologram thing upon contact with opponent weapons (not breaking the opponent weapon/armor even when passing through though, so it not separate/shredding the material molecule so super nanomolecular sharp is out even if the molecules rebond or attach back after that, and superheating the sword to melt the opponent's weapon/armor like a lightsaber is also out), and turn back in solid shape to the part that doesn't contact with metal material (preferably the weapon still intact after pass through or even some part of it stuck in that state), it may or may not pass through other material like wood, preferably not pass through flesh (it will maintain solid form to cut flesh) but I am curious if it also passes through flesh (or is that the only option the science answer can achieve).
It still maintains the shape even when it passes through so it doesn't turn liquid, melt, or spread out mist/gas (kinda...., at least in my mind it look like a mist but not spread out or still in sword form like it was a hologram) upon contact with higher temperature than this material, like gallium, and I already have this type of weapon, so gallium or similar property is out of the question.
Nanomachine/sand-like is out of the question too because I also already have that.
I want to know if there is another scientific phenomenon that can achieve this and what kinds of other effects/side effects or properties it would have, and if possible also include a way to create or shape it if it requires an unorthodox way to make this weapon.
I am not asking how to use this weapon or the martial arts, so don't bring that up including your opinion about why this sword is bad or good in a fight, after all, if your opponent knows your way of fighting, of course they can counter it and vice versa, and this is not restricted to swords, it can be made into other types of weapon including blunt weapons, I just want to make a sword for simplicity's sake.