Very interesting! Like everyone else, I couldn't summon Mutus. I would suggest making the sigils require only lines instead of being chunky figures you have to fill in, or at least make the size of the drawing point a little bigger. It took me about ten minutes to draw the sigil for mutus only to find out it didn't work. (EDIT: even after seeing your note in the comments about how it should look...)
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Thank you very much for trying the game and for the feedback, completely agree with you, we should have quantified the brush size according to the screen size, something we didn't think to do...
It should normally have worked by trying with the right sign, the difficulty of drawing it is much too big compared to the "reward" of invoking Mutus..