Petrus

ends with an 's'. English grammatical rules state that words that end with an 's' don't get ''s' after them, just the ' (Petrus' not Petrus's). Besides it just looks better that way.

English orthographical rules state an apostrophe is put after the plural -s (dog's vs. dogs'). American spelling often puts it after all s's, but "Petrus's" is still a valid option.

No. It's "Petrus's". Just the ' is a common misconception. The only place you use just an ' is for plural words ending in S and for "Jesus" and "Moses".

I vote for Petrus' sword.

It looks better.

Either is correct, Petrus' or Petrus's. Did you know that the 's is not added to words that end in z because it makes it difficult to pronounce. Eg Gomez' head exploded as the bullet ripped through it.