Darksiders

To build on Onsidic's solution:

Remember the Earthcaller? Keep this guy handy as it will be helpful and not just for opening doors. The blast of air it makes actually stuns many enemies... including... the Phantom Guard boss!

But before that, some tips:

Water: always test the waters, you never know what you might turn up. In this case, in the library bottom level there's a pool. Dive down it to receive something special.

Crows: these black birds are actually a good source of protein, green life protein that is, when you see them don't be squeamish if you're low on life, kill them ;p

Lavadudes (sorry didn't get their names): these big hulks are annoying as heck in their taunting and chasing but between levels in the library where you meet them, there's a ledge. Stay on the ledge and lure these bad boys to the edge of it. You'll be relatively safe to hack them from below or above. At least their explosion won't be a problem. ;)

Minions: and don't forget to B attack those zombie minions! B attacks shake out the life force from them giving it to you. Especially on Apocalyptic difficulty, life is important, don't miss your chances to get some if you need it!

Ok, back to business. Once past the library, you'll meet the Phantom Guard on the giant dais surrounded by vampire bat statues with one spewing bloodred liquid (or is it blood?). The guard will taunt you and come at you. This is where the Earthcaller comes in handy. Blast the guard with a cone of air and he'll stop in his tracks, giving you ample time to get a few hits in. And a few hits is all it takes to get him to fall back and call forth his foot soldiers which you might have met already outside.

Sword strokes is all that's needed to dispatch them and you can go back to the guard. I had too much fun slashing foot soldiers but I think you can ignore them if you want, try it. But either way, the Phantom Guard isn't very original nor flexible. He'll rinse and repeat until he falls in a swooned kneel.

A big B will appear above his head and following directions will net you a victory and access to the other two bat statues, which you pull over towards the center of the dais to have them spewing as well.

Samael will appear with some touching dialogue. ;p And you'll start on your next mission. Oh and get that new Shadowflight ability. ;> Good luck!

After heading into the Scalding Gallow and defeating the phantom guard, move the blocks into place and a cut scene will follow. You will then meet Samael and will also give you the shadowflight ability, this achievement and also the chasm jumper achievement at the same time.

This achievement is story related and cannot be missed.

Once you've progressed from The Crossroads you'll venture through the library until you arrive at The Scalding Gallow.

Once there you will have to battle the Phantom Guard that overlooks Samael's prison. Defeat him and move the blood spewing gargoyles into place to release Samael from his forsaken imprisonment.

You will then earn a new ability and a 20G achievement!