Herculean
Its hard conditions for the mighty work, –
Nature’s impossible Herculean toil
Only her warlock-wisecraft could enforce, –
Its law of the opposition of the gods,
Its list of inseparable contraries.
— Book 1 Canto 5
Heracles, (Latin Hercules), is the great mythological hero who had to accomplish the twelve Labours deemed impossible. He is the symbol of the work of purification/liberation as described by Sri Aurobindo, namely first and foremost the liberation from desire and ego and the perfect mastery of the outer being. (More details?)
These labours represent the goals extremely difficult to achieve, hence the adjective Herculean, which signifies a gigantic, if not almost impossible, task.
The last two verses concern the gods who represent the forces of the supermind acting in the world of forms.
In mythology, the 12 gods are paired as opposing but also complementary forces. For instance, Zeus symbolises the power of expansion which pulls the seeker and also humanity forward as fast as possible, hence his countless lovers, each liaison representing a new impulse in the yoga or a new spiritual aid. His partner, Hera, is the opposing force which tends to limit this movement so that nothing remains behind, neither in the individual seeker nor in humanity.
This is why she constantly harasses Zeus’ lovers and their children.