Sir Keegan's Tomb is the climax of the first level of Keep on the Shadowfell. Keegan is the last commander of the keep, who went mad and slew his soldiers and his family. Here the players encounter his undead remains, and after a tense conversation, they are rededicated to their mission to defeat Kalarel and close the rift.
The room at first appears to be empty save for a single coffin; however, when players investigate the coffin or attempt to leave, the coffin pops open and Keegan's bones jump out. Keegan is initially suspicious of the intruders, and the players must complete a skill challenge to convince him of their good intentions.
If they fail, Keegan goes into combat mode, and as a level 4 solo brute he's a convincing challenge to a party who at this stage are presumably level 2. It turns out he has a "necromantic burst" power, for unknown reasons, which he can fire off every turn once he's bloodied for 2d6+3 damage against everyone within close burst 3. Ouch.
In the more likely event that the players pass the skill challenge (and with a reasonably generous GM, it would take some hard-headed players indeed to fudge it), Keegan will decide to place his trust in the players, at which point he becomes surprisingly chatty.
If you're playing the module as-written, this is the first chance players will get to learn the true history of the keep. In fact, it may even be the first time they've heard Keegan's name, as although the module spends a couple of pages telling the DM the guy's life story, it doesn't trouble to give the players the same scoop.
There are some valid criticisms to be levelled at Sir Keegan's tomb as a whole. For example, if Keegan slew all his men, and then came down into the crypts to die, who the hell built him a tomb? One poster on ENWorld.org offers the compelling vision of Keegan himself being the architect, post-death, and refers to him painstakingly crafting the crypt "with his cold dead hands".
Keegan rewards players who pass the skill challenge by giving them the undead-slaying longsword Aecris. This is a great moment. Like many players, I love named weapons, and it's even better when they have a history. Not only does Aecris trace its descent back to King Elidyr of the fallen Empire of Nerath, but it is also the very weapon with which Keegan struck down his family and those under his command. It's a tangible link to the campaign world that will stay with the players long after they leave the Keep.
It can fall flat though. In our group, Keegan's dramatic presentation of the sword was followed by a silence, and the players exchanging glances for long moments, until eventually someone asked, "So, does anyone here actually wield longswords?"
Luckily D&D is a world of inexplicable magic; when our fighter grasped the weapon's hilt, the blessing of Bahamut transmuted Aecris into a much more useful bastard sword. Praise Bahamut.