I would eliminate that comma. I can't really explain why, but I would. The sentence sounds weird to me with that comma there. I think that, normally, you shouldn't put a comma before relative pronouns (who, which, whose, that and maybe others I don't recall now). Maybe there are exceptions, but I think this is the general rule.
It depends on what you want the sentence to mean. If you mean that the soldiers who stopped them were the very soldiers who had been tipped off and no others, then delete the comma. If you mean that the soldiers who stopped them were Istaran ones, and all Istaran soldiers could be said to have been tipped off, then keep the comma. I think your meaning requires the comma. It's not a matter of opini