1: of, relating to, or characteristic of Hercules (Merriam-Webster)
...leading to figurative...
2: a herculean task
(often not capitalized)
of extraordinary power, extent, intensity, or difficulty
Hercules was incredibly strong, but he was assigned some incredibly difficult tasks - which you can google yourself if you're interested; you don't need to know the details to understand the idiomatic usage.
There's also (again, from Merriam-Webster)...
of, relating to, or suggestive of the labors of Sisyphus
specifically requiring continual and often ineffective effort
a Sisyphean task
Where Sisyphus was assigned a task that was effectively impossible [to complete]. And if you don't want usages derived from Greek myths, we Brits can now use painting the Forth Bridge (for a "never-ending" task; by the time you've painted it all the way from one end to the other, the end you started at needs repainting).