I have an idea that's a bit silly. It seems that Durkon manages to destroy Kraagor's animated statue in #1214. The statue was magically animated, so it was a creature, just as much as a vampire is. Kraagor's statue has had more than 10 appearances, though for all but the last 4 of those it wasn't yet animated. Does this count as the death of a significant character for this thread? I don't think we have a precedent for this. Crystal as a golem comes close, but she appeared as a golem in 11 strips, so there's no question that destroying her counts.