I found a diamond jewel of fervor and the stats are
15% increased attack speed
39% damage to demons
50 to attack rating

Was thinking of using this in my mercs weapon as he only targets travincal council, was just wondering if this would function the same as a normal ED/ias jewel if only targeting demons, or if the enhanced damage to demons is calculated in some strange way that makes it less useful.

Damage to demon is added with all the other offweapon ED when you are hitting a demon.

So, outside your weapon that ED works like any other ED jewel that you have outside your weapon.

But in your weapon a normal ED jewel will be add that ED to the weapon ED, while the demon ED jewel will add that ED still to the offweapon ED, so in weapon is adding waaaay less damage than a regular ED jewel.

Ahhhh ok cool I was worried about something like that, thank you for the explanation!
I'll try it out in his helm and see how that goes

Yeah, it's a lot better to use it in the helm than in a weapon, as the mate above said.

better put it in a guilaumes and go to ubers

Ah perfect, my merc helm is a guilaumes anyway so I can just grab from him if ubering, ty for tip!

The bigest priority for your mercenary in uber is the survivability but if you solve the surviavbility part , this jewel will lep so more dmg out there . I used to soket it in a guilaumes or a tyrale wil doing uber for smite or frenzy barb . But is good for merc too .