OVERWORLD: SUNKEN CHAPEL OF SOBEK
You can visit this area any time after completing the first level, but because playing it is not required to beat the game, and it includes the optional DLC tombs, which not every player will have, I have saved it for last in the walkthrough. To get there, follow the elevated walkway to the southeast of the Shrine of Osiris (screenshot), jumping across the broken sections as you go. On the other side of the elevated walkway, you'll hit a checkpoint and the new location will be noted on screen. It is also indicated on the Overworld Map.
There isn't a whole lot to do here, but you will find 5 red skulls, a couple of powerups, and a few other points of interest.
Near the entrance to the Sunken Chapel is a square tile with Osiris's glyphs. There's also an interesting area with a time bomb above on the right, but you can't get past the bars just yet. For now, jump up to grab the stone crossbeam at the top of the stairs. Pull up and jump from there to the angled beam on the right. Walk down to the bottom and jump across the gap to the next crossbeam, where you'll find a red skull (1/5). (screenshots)
COMBAT CHALLENGE: Drop down and continue downstairs into the courtyard with the glowing obelisk. (screenshot) If you like, you can activate it to attempt a Combat Challenge. For details, see the Community and Combat Challenges page. Note that there is an AMMO CACHE just to the right of the scaffolding should you need a top-up during the fight.
Climb the scaffolding to find another red skull (2/5). The next red skull (3/5) is in the upper corner of the small pool off to the right. (screenshots)
Now head up the steps to the northeast. Here you'll find the ENTRANCES TO THE TWO DLC TOMBS (provided you have purchased them). When you are ready to explore inside, just activate each of the obelisks to open the doors. Walkthroughs for the Tomb of Hoarfrost and Tomb of the Embalmer are included on separate pages. (screenshot)
To obtain the red skull (4/5) in the niche between the tomb entrances, use the Staff of Osiris to raise 2 square platforms in the pool and hop across them to the skull. In the co-op game, there are no platforms. Instead one character must grapple the golden ring below the niche so another can tightrope walk across to the skull. (screenshots)
DESTROYING THE MARK OF SET: Before going after the last red skull, take a little detour in order to open up the area with the time bomb that you passed earlier. Start by standing on the tile with Osiris's glyphs, just to the left of the entrance to the Tomb of the Embalmer, and use the Staff to raise a column beneath you. Jump onto the stone crossbeam to the lower left. Move to the right end of the beam and jump from there to the round column to the lower left, then to the next stone crossbeam on the right. Again, walk to the end of the beam and jump to the next round column, then the next. (screenshots)
You should now be above and facing the entrance to the Tomb of Hoarfrost. Use the Staff to close the Mark of Set on the ledge above the door. This opens the bars back in the area with the time bomb. (screenshots)
Now let's get the final red skull. Drop down and return to the entrance to the Tomb of the Embalmer. From the tile with Osiris's glyphs, follow the cliff wall to the southeast until you're standing on top of an angled, stone wall with water flowing over it. Jump from there onto the column to the lower left, then to the next column, where you'll find the red skull (5/5). If you've found all five you will now receive the COPPER RING OF PROTECTION (Defense+). (screenshot)
HEALTH AND AMMO UPGRADES: If you have not already done so, climb up and close the Mark of Set above the entrance to the Tomb of Hoarfrost, as described above. Then return to where you first entered the Sunken Chapel area from the Shrine of Osiris. Climb onto the ledge above the bricked-over wall. Now that the bars are open, you can reach the MAX HEALTH UPGRADE and time bomb. (screenshots)
The time bomb will not fit through the doorway where the bars were, and flame jets prevent you from pushing the bomb off the right side of the ledge. Raise the Staff of Osiris to slow the countdown on the time bomb as you push it closer to the flame vent. Then use a remote bomb to blast the time bomb over the flame vents and off the edge. You can't jump over the flames without dying, so instead drop down on the left and head downstairs to the courtyard where the time bomb landed. (screenshots)
Still keeping the Staff raised, roll (or blast) the time bomb up the stairs and push it against the crumbling wall near the pillar with Osiris's glyphs. Once the bomb is in place, lower the Staff and let the bomb explode, destroying the wall and revealing a MAX AMMO UPGRADE. (screenshots)
If you've followed this walkthrough, you should now have all the collectible items in the Overworld. If you've purchased the separate DLC packs and/or the Season Pass, you can now explore the Tomb of Hoarfrost and Tomb of the Embalmer bonus levels. Again, the entrances to these tombs are indicated on the Overworld Map.
†UPDATE HISTORY: 7/10/15 - First draft of walkthrough posted.
7/12/15 - Fixed a few minor typos and added co-op notes.
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