the game is hyperrogue and can be bought on steam.
the holy grail is at the center of the round table. a well-known strategy for finding it is to draw chords with dead orbs and draw new, close to perpendicular chords at their midpoints. the chords lengths will gradually get longer until they're close enough to the circles diameter and you find the grail.

i did it without dead orbs by walking carefully and counting my steps.