No, you don't, please have a look here: https://stardewvalleywiki.com/Rusty_Key
You need only 60 museum donations to get the rusty key from Gunther. Then you can enter the sewers where you meet Krobus, that's right, but if you're talking about the Dark Talisman, you don't get it from Krobus, but he will unseal the Mutant Bug Lair (provided you already had the cutscene with the Wizard at the station), where you can obtain the Dark Talisman. Make sure to have a sword with you and some food.