You are now on a set of stairs outside the Pyramid of Osiris. There are vases here that may contain gems. Head down the stairs.
At the end of the long bridge you reach the Shrine of Osiris. After a cutscene you are attacked by a Giant Skeleton with a Sword, accompanied by normal Skeletons with Swords and Green Scarabs. Through rolling Lara should not have any difficulty to dispose of them without taking too much damage; the shrine offers enough room to evade. If you like, you can also the Submachine Gun collected in the previous level, or if you've fulfilled the Gold Point Challenge the Automatic Pistols you've been rewarded with. There is a large green health bar for the giant skeleton at the top of the screen. Once the giant skeleton falls, Lara reclaims the first Fragment of Osiris, his torso. After the work is done, use the Staff of Osiris to use the statue Lara has pointed out to learn how to use the statues as waymarkers. It points to the nearby Tomb of the Timekeeper, our next target. If you like, you can hunt around for gems, before you continue. Also, just to the right of the now closed gate that leads back to the Pyramid you can find an Ammo Cache that recharges your special ammo, displayed though the blue energy bar, in case you've already used some. After you're done, head up the short stairs where the statue pointed you. Now finally enter the tomb.