Methinks the language nerds are going to be apoplectic.

Not really. It's true that "thou" is used to indicate familiarity, which is the opposite of how it's being used here. But I keep my expectations low when reading Sluggy.

Largely to indicate familiarity, but if someone just saved your life, I think that they can be excused a faux pas or two. If you want to read my apoplexy in full, just look at my "Great war on typos" post for this strip.

Now Pete just needs to come in and say, "In the Sluggy Universe, this is how they spoke back then."

Oooh...teensy reflection in the orb on the staff.

I knew it.

Oooh...teensy reflection in the orb on the staff.

Pete, in case you're interested:

"Middle Ages English" was actually what we today call "Middle English". If you heard someone speak it, you would probably not understand a word. If you saw it written, you would probably not understand a word. What you're writing in the comic is certainly not supposed to be Middle English (unless you literally don't want any readers to understand what the characters are saying).

Modern English began a hundred years or so before Shakespeare's works. I think THAT is what you're trying to emulate, something like Shakespearean English. Even though it's modern English, it's very old, archaic modern English. We can understand it, mostly, but some words and usages are out of date or even unknown.

The use of the pronoun "thou" (and "thee", and "thy") is a case in point. It is just the singular of "you", what you use in addressing one person. It's also the familiar form; if you called the king "thou", you might run the risk of losing your head. With kings and other powerful, high-class folk, you have to use the "polite" plural form "you". Because, you know, they're worth more than you are.

That said, an ignorant commoner unfamiliar with the intricacies of grammar and usage might well use "thou" to address someone of higher class or rank, just because s/he doesn't know any better. For exactly this reason, JRR Tolkien imagined Merry Brandybuck speaking to Théoden, King of Rohan, using "thou", with Théoden being amused by the hobbit's overly familiar usage.

Using "thou" is tricky if you're not used to it, but once you get the hang of it, it's easy and fun. Here's a retranslation (so to speak) of the comic:

PANEL ONE [no changes] [I don't know what "Grammercy" means, but I assume it's a name or title or something like that. Unless it's some French-derived term meaning "thanks".]

PANEL TWO
Wizard: Wist thou who I am?
Girl: Thou art the wizard Yffi! Tales of thee wot I from my grandfather! [or, At my grandfather's knee wot I tales of thee!] Prithee forgive my gaze upon thee!

PANEL THREE
Verily, all who behold my face shall a terrible fate suffer. For this very cause keep I my face in shadow, as best I can. Yet thou hast nought to fear from a sideward glance. Regardest thou me a monster?

PANEL FOUR
Girl: The tales have it that thou wieldest mighty magic sometime for good, and sometime for evil.
Wizard: Which believest thou?
Girl: Methought thou mightest of thyself tell me.

A few grammar notes.

Panel 2:

I'm not completely sure that "witen" is an appropriate verb here. ("I wot", "thou wist") "Witen" is for knowing facts rather than e.g. being familiar with people. Maybe some German speaker could weigh in. I am sure that "know/knowest" would work fine. But using "wot/wist" here sounds good.

You wouldn't say "whom I am". Copulative verbs like "am" take the nominative (subject form) on both sides, so you wouldn't use "me" or "him" or "whom".

By the same token, you wouldn't say "tales of thou" or "looking upon thou". You would say "tales of thee" and "looking upon thee". Think of I vs. me. Thou vs. thee is the same thing.

Panel 3:

"Any whom look" is wrong; should be "any who look". Also, "Doth thou" should be "Dost thou"; "doth" is third-person singular, "he doth" and "she doth" and "it doth". (This is when "do" is used as a helping verb. If you use it as a standalone verb, like "what are you doing?", you would say "thou doest", "he/she/it doeth".)

Panel 4:

"Doth thou" --> "Dost thou", as above. Also "thou mightest" rather than "thou might". And if she's using "thou" all the time, she would probably say "thyself" rather than "yourself".
but
Hope that's helpful rather than pedantic. (Or helpful even if pedantic.)