"at will" vs "by will"

My friend asked me the difference between these two expressions: at will and by will. I’ve found only at will in my dictionaries. Now I am interested weather it exists. If so, what is its meaning?

Hi Inga

I understand ‘at will’ to mean something similar to ‘whenever one wants’.

You may see ‘by the will (of)’, for example, used to mean something similar to ‘according to the desire(s) of’. The word ‘by’ might also be part of a passive construction.
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Thanks a lot. That was useful.