Hint: Life Upgrade
If you missed the life upgrade that is in the Clock Tower (in the room where you are getting Spike Beasts thrown at you), you can still get the upgrade later in the game by using the Scorpion Sword. After you get the sword and leave the library, you will break through a wall which leads into the Clock Tower. Instead of heading down towards the central chamber, head up and you will eventually reach the room where the Brute was throwing Spike Beasts at you. It doesn't throw them anymore but you can now break the wall on the left by using the Scorpion Sword.

Unlockable: Light Sword
The light sword is the most powerful secondary weapon in the game. When in the mystic caves you come upon a door that you must unlock via switch. Directly after that door there is a drop in the floor. Drop down and break the left wall and break the rack to pick up the light sword. As with all secondary weapons it is unbreakable, but if you throw it you lose it for good. The light sword has maximum damage and maximum attack rate.

Unlockable: Pelican axe
In the big room with the hanging gardens and the four levers that have to be activated in order to move on the first time you enter: later in the game you will find a way to the top floor. You will be chased there by the Dahaka in the present. As usual he will chase you all the way to a time-portal. As you're wall-running through the center-aisle leading to the hallway to the time-portal (trust me, when you get there you won't have any other places to go, you'll know what I'm talking about), notice how the camera turns in front of you and shows the Dahaka get angry and smash a wall. Once you have made it to the time-portal, don't go back to the past, but return to the center-aisle and the wall the Dahaka smashed. You can now jump straight through it. Activate the switch to get the big brick out. Use the brick to get on top of the rail above. Use the rail and continue to the wall to the right. Drop down when you reach that little spot with those ninjas and those transparent Blade Dancers. After you've beaten them, break the wall. There's a weapons-rack in there with a pink weapon shaped like a pelican. It's one of the miscellaneous secondary weapons.

Unlockable: Secret Ending
To get the secret ending simply obtain all 9 life upgrades, when you get to the room before the Empress' throne room there will be a new sword waiting for you. It is the water sword and said to be the only thing that can kill the Dahaka. For your final battle, instead of fighting the Empress you fight the Dahaka.

The Dahaka’s attacks are repetitive and predictable. When far away he will shoot his tendrils throw the ground up at you, run around in circles to avoid it making your way ever closer to him. If you get too close he'll shoot out his tendrils directly at you, roll to dodge them. As you roll slow down time and go in for some heavy combo hitting, as you get him lower in health the Empress will help you and shoot him off the arena, he will be stuck on the ledge, stock up on sand tanks (They regenerate around the arena) and slow down time or use Ravages of time to inflict maximum damage. He'll eventually get back up and jump at you, constantly roll until he calms down and remains stationary. Repeat the same method as before until he is defeated.