While this sword may not be the most elaborate, it is certainly the most functional.
Magical runes adorn the blade, a small compartment and button are secreted away from prying eyes in the hilt.
This blade counts as magical for the purposes of overcoming magical resistances.
There is a small hidden space within the hilt for a vial of venom, during combat the user can press the button on the hilt for a bonus action to secrete venom all over the blade, the venom then persists for the next 1d4+1 attacks as opposed to the standard 1.
The venom needs to be one that afflicts the target by contact.
Proficiency with a rapier allows you to add your proficiency bonus to the attack roll for any attack you make with it.