Enemies doing astral damage and question about "chaos" element

[Zafarion]

Interestingly, there are 2 bosses that use astral spells, but this element is not listed in the char resistances section and I haven't found any ring/cape with astral resistance. Does it always count as zero? Maybe that's why people complain about the last boss, who uses astral attacks.

Also there is supposed to exist the "chaos" element and the Spellbinder supposedly uses it, but until now I thought it was the same as the VOID element. There are CHAOS along with VOID or only VOID? I'm confused. If this CHAOS element exist, then is the same case as ASTRAL (not listed in resistances section).