The lowest I can think of, besides ETCR, that is appropriate for the party would be the Catacomb of the Scion from Libra Mortis, page 187. It is suitable for 13th level characters yeah, but feel free to pull punches if you're worried they aren't fully ready for an on the nose challenge, but keep in mind that every game starts like Record of Lodoss Wars and becomes Slayers once the party tries and floods the entire tomb by diverting a river into it because of a few ranks of Knowledge (Architecture & Engineering)