Fraxinus
chunky plant goop
That's such a shallow design choice considering Demise was implied to look different to all who saw him. Groose said something that made it pretty apparent he saw something vastly different than Link/the player did. And saying what Link/we saw is the objective appearance is absolutely a waste of a strange concept like that, and it's totally pointless to even say he looked different to each person if there was an "objective" form only seen by Link in that he resembled Ganondorf run over by a steam roller and set on fire. Sounds like an effortless attempt to connect Demise specifically to Ganon in his allegedly "fluid" appearence, but that doesn't mean his other appearances didn't resemble something or someone else also depicted in the series as an antagonist.There is also the fact that Hyrule Historia has a quote from the person who designed Demise who says Demise was created to intentionally resemble Ganon/Ganondorf.
Also, RE: light force, it's possible it refers to the essence of Hylia in the case of the force within Princess Zelda and therefore may or may not mean those who seek her out for her life in that regard could be linked to whatever curse set in motion by Demise. Ghirahim sought Zelda out for the same reason. Bellum arguably wanted Tetra for her life/light force as well. There's something specifically unique about the essence within the Hyrulean royal bloodline and that would supposedly be its link to Hylia.