Custom Demon ~ Karra No Su

Name: Karra No Su

Abilitys: when the user is inraged the demon takes control this deamon has 4 stages
The user appears with black aurora surronding them and their Taijutsu power is extended , also some minor attacks deal no damage unless it is used more than onece (eg. a academey student using a average punch) the user gains the ability

Downsides: the user loses battle mind,after use the user is unconsious and needs medical treatment , the user is slow while in the Karra No Su form , The users loses the ability to perform any genjutsu , If the user is inraged in a dream through sleep the user gains the Karra No Su form and attacks any one in sight without the user noticing anything , the users does not have any memory of times they are in Karra No Su form

history: there once was a beast which would kill any weak-minded person in 7 days unless it was sealed , The privious hokage found the perfect host which was Danny Furru and sealed the beast in his back costing his life

Denied, one, your grandfather has to be very strong and only a select few know how to seal. Since so far your grand father is not of great importance to our story line he would not know the sealing jutsu.
Two, your downsides are the same as most other common ones used for jutsus, kekkei Genkai, etc...
Three, there isn't supposed to be stages to a demon seal.

edited

There aren't supposed to be stages. Also, you need a longer history.