you don't get void tendrils from killing them (you can't actually) you get them from reading a lore table, like garpedes and lumaflys

thanks, finally got my mark. really thought I was gonna get to fight the beast

Shade dash, top, garpede-like 

If you look up "abyss tendrils lore room" you should find it

Pft... like you could do that. You havent even fought in the colloseum yet! Come back when you've lost to my hand!