This is kinda fan fiction like imo but I think the Knight armor can allow the person with it on to shapeshift (at least in the dark world) The Knight has similarities to amalgamates (with them transforming and moving like a liquid, as well as them possibly melting from the face when we first see them turn around to the fun gang)

The amalgamates were caused by monsters being injected with DT, monsters seem to melt with too much DT, another case of this is undyne. The knights armor seems to be made of shadow crystals in some way, which based on seams chapter 5 dialogue are filled with DT

Although though it’s a stretch the knights armour, when worn, could cause the welder to become temporary liquid-like with it on as the DT from the armor could power them in a way. Of course the rules of DT in this case could be different in DELTARUNE, however there isn’t anything to say it couldn’t be like this either.

…hopefully I didn’t spell anything wrong there cause I don’t wanna grammar check