Endoribonuclease that cleaves tRNAs and rRNAs (PubMed:29563550). Cleaves tRNAs 11 nucleotides from the 3'-terminus at the acceptor stem (PubMed:29563550). Does not act on tRNA(Sec) (PubMed:29563550). Can also use Mn(2+). Shows a pseudo-dimeric U-pillow-shaped architecture of the SLFN13 N'-domain that may clamp base-paired RNAs. Belongs to the Schlafen family. Subgroup III subfamily.