Runefang 8th edition warhammer empire magic weapon overview
The runefangs areone of the more iconic magic weapons (after the hammer of sigmar) in the warhammer world. Twelve runefangs were made by the dwarf runelord Alaric the mad. They were forged to cement the alliance between men and dwarfs in Sigmar's time. They were given to Sigmar's twelve cheiftains and past down through the ages to what are now the elector counts.
The runefangs are powerful weapons wounding automatically and allowing no armour save against wounds caused. Several special characters in the empire army also carry them, these are the emperor Karl Franz himself (though he can instead elect to use the hammer of sigmar for an additional points cost), Kurt Helborg and the unpredictable Marius Leitdorf. This makes it I believe the only magic item that is allowed duplicates in 8th edition warhammer. It is also worth noting none of the special characters use shields so reducing their armour save slightly.
The runefang was undergone a few changes in 8th edition essentially the same in combat it has a reduced points cost and is available to all lord choice now rather then just a general of the empire. These seemingly small changes make it much more useful and actually worth taking.
Pros and cons
Pros are that the runefang is an excellent weapon there are not many better out there (with the except of the hammer of sigmar and dwarf rune weapons) auto wounding with no armour save is effective in nearly every situation. Aside from the special characters the best character to take a runefang is the grand master as having 4 attacks over 3 for a general or 2 for a arch lector makes it that much more effective. There are also a few spells that enhanced the effect most notable is the beast lore spell savage beast of horros which gives the target a extra 3 attacks. Combined with a runefang this can make the character wielding it particularly deadly. A nearby celestial hurricanum with its +1 to hit bonus is also very handy.
The con side of runefangs is that it is expensive leaving you few points to take defensive magic items and armour or other items. If a grand master has taken it though he comes with a 1+ armour save that will make him all but imperious to low strength attacks. Empire characters are not the fastest out there initiative wise so can be vulnerable to quicker enemy characters. Another con is that once your opponents realise who has it they are likely to become the target of a lot of attacks this is particularly true of the special characters.