Sounds more metaphorical really. Aatrox's actions created Trynd as he is now, else he wouldn't be like he is.
I believe that line is simply referring to Tryndamere's ult. Tryndamere first used it (in lore) when Aatrox attacked his village, and Aatrox is probably the source of Tryndamere's abilities.
I would say unlikely as it doesn't visually fit the thematic of all the other Darkin weapons. They tend to be a red/black theme. Trynd also doesn't treat his weapon any differently from other general weapon wielding champs. Pretty sure the line is referencing how Aatrox is the Darkin of War and Tryndamere is a warrior leader who fights to the death, and then fights in spite of it.
I don't think all darkins are weapons. I think rhaast is just a unique case. https://youtu.be/3Ny4piDu4W0?t=9m7s at 9:07 that line implies that rhaast was captured and put into the weapon.
Pretty sure demonblade skin of his is an alternate future when aatrox is passed on with tryndamere as its host.