I created a weapon with a similar ability back in the early 3.0 days. It's simply a 1/day function, rather than happening on a crit. The effect relies on a metamagic feat I designed (since re-created by other various d20 sources), Eye of the Storm, which creates a safe zone inside an area effect spell, to allow for point-blank fireballs and the like. (And a Warfork is a polearm from Bastion Press' Arms and Armor sourecbook.)
The Black Halo
This +2 Unholy Flaming Burst warfork burns with pale blue light and a thin, greasy black smoke. Once per day, the wielder may utter a command word to unleash an explosive ring of dark azure flame extending 10' on each side of his position. The square the wielder is standing in is not affected. The flames do 9d6 damage (REF save for half.) Half of the damage is considered to be divine, affecting even those creatures immune to fire.
The Black Halo only functions as a +2 weapon when used by anyone of good alignment, though the negative level from the Unholy ability still applies.
Strong evocation [evil]; CL 12th; Craft Magic Arms and Armor, Eye of the Storm, flame strike, unholy blight; Price: 91,440 gp; Cost; 45,720 gp + 3658 XP.