Caizhou (786) — the same event in Old Book of Tang, New Book of Tang and Zizhi Tongjian

Old Book of Tang: Li Xilie was killed by his subordinate Chen Xianqi, who took Huaixi with him and surrendered.; New Book of Tang: Li Xilie was…

What each source says

Li Xilie was killed by his subordinate Chen Xianqi, who took Huaixi with him and surrendered.

四月丙寅，淮西李希烈為其牙將陳仙奇所ＣＧ，並誅其妻子，仙奇以淮西歸順。
Location: 舊唐書卷12 貞元2年 4月

Li Xilie was killed by his subordinates, ending the Huaixi rebellion.

丙寅，希烈伏誅
Location: 新唐書卷007 德宗·順宗·憲宗貞元2年4月

Chen Xianqi sends a physician to poison Li Xilie.

夏，四月，丙寅，大將陳仙奇使醫陳山甫毒殺之。
Location: 資治通鑑卷232