This should be a +2 weapon.

Vicious blade of sharpness?

I would say the plus 7 should apply to any attack, and go to 14 on a critical

Quote from Yasha_Zervkric >>

I would say the plus 7 should apply to any attack, and go to 14 on a critical

why? It's already rewarding you for scoring a critical hit. Along with the PHB critical hit rules of  "Roll all of the attack's damage dice twice and add them together". Unless you're running a special variation of the rule. So in this current example it would be:

If the weapon would be as you say, it would most definitly require attunement to make it balanced.

How would changing the damage bonus from a flat +7 to a +2d6 affect the overall power level?

this should be on any critical instead of just nat 20s, that way the weapon isn't only cool 1/20 hits

This doesn't look that powerful. Why is it Rare?

Its does not say natural 20 ?  On a roll of 17, plus str mod +3 would be a 20 on your roll, you get to add 7 if you hit the target.  On a roll of a crit 20, it would fall to weapon damage dice again, +7,  This is our House rule, 

I think it's intended to be what shows on the die face.  By your logic, if I rolled an 18 with a +3 str, then my roll would be 21, and not a 20, so no +7, as it does not say a roll of 20 or more.  

My favorite warhammer.

I have never made a critical hit since I've never rolled a natural 20 on attack, so I'm not sure this would be of any value to my fighter lol.

Why is there no fish under applicable weapons? It said all weapons . . . I WANT TO SMACK PEOPLE WITH VICIOUS FISH

(Yes this is a legitimate complaint) 

Quote from Bubben246 >>

How would changing the damage bonus from a flat +7 to a +2d6 affect the overall power level?

I'd think that's fine, as it already only happens on a crit, so the extra dice aren't getting magnified by the crit itself.  You'll be sad when you roll that "2" and your Rare weapon that *only* works on a critical hit (5% of the time) did half the damage of a comparable vanilla "+2 weapon".  But power level... yeah, that's about the same.

Personally, I'm confused about why the description didn't say "when you score a critical hit".  Did they feel the need to shaft the Champion subclass just a little more for some reason?

This is an interesting power, but I'd take the "+2 weapon" every time instead of this, given a choice.  I don't need +5 damage 5% of the time (7 vs 2), I need to *hit* 10% more (+2 to hit)!  It makes a good power to give a BBEG weapon that won't really shift the balance of power when your PCs get it!

Just because you mentioned this, now the weapon Vicious Fish. Will ne added to my campaign

Quote from KanedaSyndrome >>

I have never made a critical hit since I've never rolled a natural 20 on attack, so I'm not sure this would be of any value to my fighter lol.

 It's a magic weapon so it overcomes resistance to non-magical slashing/bludgeoning/piercing, so it's not completely without value. 

Does this effect trigger from a rolled 20 on the dice or after the modifiers have been applied? let's say they rolled 18, +2 to hit = 20.

I'm a new player and I don't own a copy of the PHB yet, but my interpretation of what's written is that you roll a Nat 20 (maybe at the DMs discretion any form of crit on the dice) and then the +7 damage is added as an extra, so whatever you roll for damage (including the critical damage die), Strength/Dexterity modifier and THEN the 7. Otherwise the weapon has no point is you'd potentially hit another 8 or 10 damage from the crit roll, just to be replaced by a comparatively meagre 7. Just my two pence!

My question:
Normally a thrown 20 means a crit. Would it therefore be conceivable to change the passage with the thrown 20 and say that a crit causes the extra damage. I have to think of a champion who crits at 18-20.