Imperial path (green on the graph):
Is indeed something like a "hard(er) mode" for the game. In most cases, enemies will rush at you right away, coming not only in great numbers, but in great unit variety:
You can also use up to 3 offensive casters here (but only one healer, compared to 1-3 on the Light Path):
So, using AoE magic to actually kill some units during one turn is possible:
And OFC, you can switch to 2 remaining paths on this path, by proper dialogue choices and "final" choice during the "acquire the sword" mission #40 (mos def the hardest one so far, I had to restart ~5 times to beat it).
A) Staying on Imperial path:
B) Moving onto Independent (or later Chaotic) path: