I really like the Sealed Sword's charge attack mechanic: it randomly selects an ailment (fire, shock, poison, stun, freeze, curse) and inflicts it on the target. What if a third line were made that emphasized this effect?
4*: Lambent Blade
A powerful sword of mysterious origins whose magical seals have been tampered with. It emanates raw elemental power.
Appearance: Like Sealed Sword, but the outline shimmers light blue, yellow, and orange, fading in and out.
Damage: split normal/elemental
Regular attack: Wide swipe like a regular heavy sword, followed by a forward thrust with narrow cone of attack but high knockback and fast recovery.
Charge attack: Slams sword forward in a vertical chop, sending a glowing fissure in the ground forward a few tiles. Monsters standing on the line will take damage, have a mild chance (~33%) of receiving a moderate status effect, and experience mild knockback away from the knight. The glow color would be based on the status being inflicted: fire, shock, freeze, stun (no poison or curse). The closest visual I can think of is Cloud's Blade Beam in Advent Children.
5*: Radiant Broadsword
A legendary blade whose magical seals have been forcibly removed. It crackles with the power of the elements.
Appearance: Sealed Sword minus outline; crackling blue, yellow and orange sparks along its length (like the way a Storm Driver looks)
Damage: split normal/elemental
Regular attack: same as above
Charge attack: same as above, but with three lines instead one. Still moderate strength effects, but maybe upped to 50%
No offense, but it seems like you're suggesting another Divine Avenger? And it sounds a bit overpowered. The forward thrust and the quick recovery sounds like a mix of a Piercing blade and a heavy sword. I mean, I've read quite a few threads suggesting a neutral sword line for the Sealed Sword, but don't you think this is just another Divine Avenger? Except it inflicts statuses?