There are two main plot points where you have to make a decision.
1) Emperor’s Offer
If you choose to side with the Emperor you will receive a sword called Discord and continue with the story.
If you choose not to side with the Emperor you will be thrown into the abyss and have to fight your way out to continue the story.
2) Seraph’s Offer
If you choose to side with Seraph the game ends.
If you choose not to side with Seraph you must defeat him before the story comes to an end.
Some tips for anyone who plays this game
Hard routes tend to unlock shit that makes the game easier long-term. Grim mode (a set of increasingly difficult monster-hunting stages) and your first ruby are both attained by choosing more difficult options. Also the game likes to calibrate your mouse DPI to be higher than appropriate, making the controls sluggish and frustrating. Tinker with mouse sensitivity manually to get it to a point where it feels responsive.
At the right angle and with enough force you can knock weapons away from opponents (and vice versa). As long as you’ve got inventory space, you can catch dropped weapons by holding the left mouse button as you touch them. This includes thrown projectiles. If you end the round still holding them they’ll be added to your arsenal permanently. Many of the game’s best weapons can only be gained in this way.
Socketed weapons can be enhanced with gems. There’s a bunch, all with different effects: rubies heal you on a strong hit, sapphires can freeze and shatter enemy weapons, and so on. There’s even one you can trigger to let you ‘phase’ through enemy weapons. You have the opportunity to get several gems over the course of the campaign, and gemmed weapons stolen from enemies retain their properties, but the chief source is Grim mode.