Hi,
There is what I received :
> I have two questions regarding Actions of two units :
>
> Harpies (AoU #24)
>
> Could I use it's action to discard multiple times if I could restore
> them by some effect ? (by Chillwind for example)
>
> Alluring Demonettes (Warpstone Chronicles)
>
> Could I use their ability multiple times in one combat or only
> once ? Is it a triggered ability or "when attacking" is only a
> requirement ?
Hi,
Regarding the Harpies, you cannot re-use the ability multiple times in
the same quest phase, even if you use chillwind. This is because it is
a response action, which is based on a specific timing trigger. These
effects can only be used once per trigger per copy of the card in any
given turn.
The Alluring Demonettes is the same thing.
Hope this helps
James
Thank you James for the answers ! 
Cain