Info (Spoilers Ahead!)
Lilah Morgan first encountered Angel when she purchased him from two brothers that had Angel held in a gladiator like underground arena. Lilah, an associate at Wolfram and Hart, offered Angel his freedom in exchange for working for Wolfram and Hart. Angel declined and he was sent back. Lilah worked closely with Lindsey McDonald and sometimes with Lee Mercer to in Wolfram and Hart's efforts to thwart Angel's attempts to save the world.
After bringing back Darla and Drusilla, the two vampire massacred en entire department of Wolfram and Hart lawyers, leaving only Lindsey and Lilah alive. Both were promoted and told that their progress would be evaluated soon. The two were constantly on edge and while working together, constantly trying to best each other. When they were being evaluated they chose Lindsey but he decided to leave, not only giving Lilah the job but also saving her life.
Lilah continued to work hard, clashing with Gavin Park and answering to Linwood Morrow. When Wesley was split from Angel Investigations Lilah attempted to get him on Wolfram and Hart's side. She baited him, toyed with him and tested him. One night, the two ended up in bed. Though both were on opposite sides, their affair continued after Angel's son Connor dumped him to the bottom of the ocean in a box. Despite some failures at work, Lilah went to the senior partners to discuss Angel, believing Linwood to be ineffective. The senior partners agreed and at a board meeting, as Linwood was about to get rid of Lilah, she revealed that she'd be taking his job and he was decapitated. Now in a position of power Lilah tried her best to find out what happened to Cordelia after ascending to a higher dimension while still carrying on her affair with Wesley. She and Wesley also began to develop feelings for each other.
A creature known as the beast rose in the same spot where Connor was born and went on a rampage at thw Wolfram and Hart Los Angeles office. Everything and everyone was destroyed except for Lilah. Wesley, after breaking up with her, went back and helped her escape through the sewers. He told her to disappear and change her name. However, she stayed in Los Angeles and when Angel was turned into Angelus she went to him for information since she wanted to get her life back. Hiding out at the hotel with everyone from Angel Investigations, Angelus escaped and chased Lilah and Cordelia through the hotel. The two women were separated and when Lilah found Cordelia, Cordelia stabbed Lilah in the neck, killing her (Cordelia wasn't really Cordy, she was possessed). When Wesley, Gunn, Fred and Connor found Lilah, Angel was holding her and drinking from her. They decided that would need to cut off Lilah's head so she wouldn't be turned. Wesley offered to do it, grieving as much as he could for her down in the basement before completing the act.
However, this was not the end of Lilah as her contract with Wolfram and Hart extended past her death. She was brought back to hand over the Los Angeles office of Wolfram and Hart, rebuilt now, to Angel Investigations. She gave Angel his own tour of the new offices and found Wesley in the basement trying to destroy her contract and set her free. The contract remained and Lilah is still under contract with Wolfram and Hart.