Sword Of The Dragonslayrer [OC]

The basic dnd Dragonslayer sword is very boring so I decided to make my own.

A sword carrying the spirit of the dragonslayers that have wielded it through the ages.

• requires attunement
• longsword 1d8 magic damage + 3 to attack and damage
• all attacks, made with the sword, that hit a dragon are critical hits.
• the weilder knows the type and location of every dragon within 60 feet (useful because of the dragon polymorph ability)
• weilder gains the evasion ability against dragon breath attacks
• weilder is immune to the frightful presence of dragons

Please suggest changes that you think could make it better/more ballanced

edit: Turns out this is way to op I am currently running a campaign where the players are tasked with finding another magic weapon that is cursed. In order to break the curse the Arcanaloth that did the curse has to remove it himself. The Arcanaloth is going to ask for the above mentioned sword of the dragonslayer for breaking the curse. The sword is currently owned by an adult bronze dragon which might just give the players the sword, (depending on some very good persuasion checks, but I'll see how this goes) since their cursed sword is going to be used to permanently kill a Nalfeshnee which means to open a gateway between the abyss and the mortal world to flood it with demons. However if the players manage to take the sword without the dragon noticing he is going to be killed by the Arcanaloth.

The sword must then be powerful enough for a CR 12 Arcanaloth to beat a CR 15 Adult bronze dragon without to much effort