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I'm not sure how to resolve some incendiary or grenade attack.
Who makes these grenades? Historically, the secret of 'Greek Fire' was so jealously guarded that all knowledge of its manufacture has been lost to history. Something similar would probably apply to 'Emphidian Fire'... (See the comments about such things only being prepared by Sorcerers or Elementalists - I feel that would be likely in the world of Legend.)
Grenades were a relatively late invention (and somewhat unreliable); they only appear around the late 17th and early 18th Centuries (although cast iron 'bombs' where known in the late 15th Century). Whether these things exist in your world of Legend is up to you. Personally, I've avoided them.
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Maybe a grenade can have a weak FIRESTORM effect
'Firestorm' is a very powerful spell - Level 9 - which blasts anything not protected by magic. If you really want to use grenades, I would suggest giving them a damage rating (Xd6), a blast radius (X metres) and have the damage reduced by AF. Note that 'grenades' until the 19th century would be awkward things, made of easily-broken pottery and lit by a fuse; so the user would first need a source of fire, then they'd need to light the fuse, then they'd need to fling the grenade/bomb (q.v. missile rules for 'rock')...
The bigger the damage rating and blast radius, the bigger the bomb (and the more it would cost). As with guns, I would suggest such items also have a 'misfire' chance (they may not go off, the fuse might go out, they may explode in your hand).
See also Wimlach's comments about fragile pots. While black power will not explode without a spark (i.e. a lit fuse), the container could easily break, it can be affected by damp, &c. If a character wanders around with fragile black powder grenades/bombs they're going to have to take good care of them and decide how they're stored.
Anything that's designed to ignite on impact (e.g. Emphidian Fire) would naturally explode if broken...
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no knock down rules now are there?
No, there aren't.