Minecraft wiki entry for Spawner_traps, associated text: A lava blade trap consists of using water underneath the spawner and funneling these into a single line. At this point, the mob should find themselves approaching a sign at ground level and a 'lava blade' above it which flows over the top of the sign, but no further. This allows the mob to drop items at their feet for your collection. A tutorial can be found here: https://youtu.be/EcOySVPf_t0 .