I decided to do an Ed the Undying run, but I ran out of HP. I clicked my handy campsite link, and no can do, no tent for me to nap in. So I go to my pyramid, and...well, I don't know where to go from there.
How do I heal?
You die! (Ed the UNDYING, get it?)
Ed is immortal. Anytime you would normally lose a fight, like running out of HP or combat taking more than 30 rounds, you are magically transported to the underworld, fully healed, and most negative status effects are removed.
After completing your business in the underworld, you will get a choice of going back to the fight or returning to your tomb (and losing the turn you spent fighting unsuccessfully). You can return to the fight twice for free, after that it begins to cost increasing amounts of Ka to fight again.
You can also purchase several kinds of bandages for Ka in the Everything Under the World shop, located in the underworld. These bandages can be used to restore Ed's HP as well, though losing a fight is often easier (and happens naturally).
If you have no bandages and are at 0 HP (so you can't initiate a fight), you can get to the underworld through the portal in the Small Pyramid. Taking the portal costs 1 turn to come back, so it is almost always better to use a single linen bandage to get a few HP back, then lose a fight to get fully healed (and get back to the underworld to buy more bandages), then win the fight to earn back some ka.
This "coming back from the dead" mechanic is why you have to fight Ed so many times when you do the MacGuffin quest in a normal ascension.