Very rarely a puzzle in a video game becomes unbearably frustrating and Peculiar Poem is one of those puzzles.
Most of the treasure maps (Tunnel in the Cliffs Map, Drowner’s Map, Vidlak Bandit’s Map) are difficult but full of details, and the Peculiar Poem is one of the few providing only a series of directions with no starting point.
You need to get your hands on the poem if you meet Bolek on the road in the second Kuttenberg region.
This poem was originally given to Bolek, a well-intentioned treasure seeker, by a drunken monk who told him it contains the directions to the Holy Grail — the cup employed Jesus at the Last Supper, and from which his blood was believed to have been shed during his crucifixion.
Holy Grail location in Kingdom Come: Deliverance 2

From two magical peaks, that gaze into the heavens, like a thirsty pilgrim I set out on the path. Across a valley overgrown to the very edge of the earth, there I shall drink from a spring that will not let me perish.
Bolek is seen digging a hole during your first meeting with him. He is in search of the Holy Grail, but he agrees to hand over the poem to you. You can meet Bolek three times in the game so even if you miss the first meeting you can some across him two more times.
Go through the poem at least once to understand the location of the treasure. All clues are hidden within the poem but of course, you can just read this guide to easily find it.
More specifically, the Holy Grail is hidden near Sedletz Monastery in the far-eastern section of the Kuttenberg region.
To this end, “two magical peaks” refers to the two spires of the Sedletz Ossuary in the cemetery.
From there, you will head south through the open fields to find the valley “overgrown to the very edge of the earth,” which is indicated by a wooded, serpentine river at the map’s south edge.

Right in the middle you find a “spring” full of bathtubs and clothes washing. You will also find food and blankets all over the place. Beyond this is the next box that is busted open and floating in the water.
A golden chalice rests next to it — congratulations, you’ve found the Holy Grail! And if you require any more proof, check out the nearby mermaid skeleton, grasping a sausage fork plucked from the water.