It doesn't translate to "the undertaker", I meant that it's a nickname like "The Undertaker" is a nickname. Or "The Red Baron", "Jack the Ripper". That is to say, a nickname that conveys notoriety/fame rather than a nickname for your chums.

From the Demon's monologue the end of the episode:

"I see. I remember now. It was Frieren. The mage who contributed significantly to humanity's analysis of Zoltraak and slew more demons than anyone else in history. Frieren the Slayer (葬送のフリーレン). One of the geniuses I despise."

So we see that in the Crunchyroll translation (and I imagine the translation of the manga as well), 葬送のフリーレン gets 2 translations:

• Title of the show: 葬送のフリーレン ➡️ Frieren: Beyond Journey's End
• Nickname of the character: 葬送のフリーレン ➡️ Frieren the Slayer