Magnus's or Magnus'?

It's already come up: how do we make "Magnus" possessive. If you read the googled excerpt below, you'll learn the traditional Magnus's and the alternative Magnus' are both acceptable. The traditional "Magnus's" will work because that how we'll say it: "Magnus-es."

Singular nouns ending with an “s” or “z” sound:

Many respected sources have required that practically all singular nouns, including those ending with a sibilant sound, have possessive forms with an extra s after the apostrophe. Such sources would demand possessive singulars like these: Senator Jones's umbrella; Mephistopheles's cat. On the other hand, some modern writers omit the extra s in all cases, and Chicago Manual of Style allows this as an “alternative practice”. Generally, Chicago Manual of Style is in line with the majority of current guides, and recommends the traditional practice but provides for several exceptions to accommodate spoken usage, including the omission of the extras after a polysyllabic word ending in a sibilant. Rules that modify or extend the standard principle have included the following:

* If the singular possessive is difficult or awkward to pronounce with an added sibilant, do not add an extra s; Such sources permit possessive singulars like these: Socrates' later suggestion; James's house, or James' house, depending on which pronunciation is intended.
* Classical, biblical, and similar names ending in a sibilant, especially if they are polysyllabic, do not take an added s in the possessive; As a particular case, Jesus' is very commonly written instead of Jesus's – even by people who would otherwise add 's in, for example, James's or Chris's. Jesus' is referred to as “an accepted liturgical archaism”