Lore of metal in warhammer 8th edition
Metalshifting is the name of the lore of metal's lore attribute, it means that magic missiles and direct damage spells from the lore of metal have a "to wound" roll that is the same as the model(s) hits armour saving throw to a maximum of 2+. It allows no armour saving throws and is also a flaming attack.
This lore attributes makes a wizard with the lore of metal deadly to heavily armoured troops as are found in the empire, warriors of chaos and dwarfs or any army with heavy cavalry. It is also useful against monsters with a decent armour save.
Searing doom is the signature spell from the lore of metal, it is a magic missile spell that causes d6 hit that use the lore attribute metalshifting to cause damage. There is also a higher casting version of the spell that inflicts 2d6 hits. Of all the signature spells searing doom shares the highest casting value alongside wyssan's wildform from the lore of beasts.
Metalshifting makes this deadly against heavily armoured troops such as heavy cavalry or even monsters such as dragons and treemen (particularly as treemen are also flammable)that have a good armour save. It is also flaming so negates regeneration against dark elf hydra's for example. Against lightly armoured foes though this is an expensive spell to cast. Definitely a spell to take if you suspect you will be facing powerful heavy cavalry or a monster it will be effective against.
Plague of rust
This hex spell lowers the target units armour save by 1 point permanently reducing its armour save for the rest of the game. Combined with either the armour piercing boon of enchanted blades or transmutation of lead temporary lowering of armour save (or indeed both) it can lower the target units armour save by 2 or 3 points in one round (or in fact over several magic phases). This can make a lot of difference to low strength troops who would otherwise struggle to get through heavy armour and swing combats massively in your favour.
Be careful using this spell if you intend to use searing doom or golden hounds as it reduces the targets armour save permanently, it also therefore lowers your chance to wound with these two spells. Its is best to save until the target is engaged in melee combat unless you have a lot of ranged attacks shooting or magic missiles that it will make a big difference too.
Enchanted blades of aiban
Enchanted blades is an augment spell that grants +1 to hit with both hand to hand and shooting attacks. It also makes them magical and grants them the armour piercing special rules. This can allow you to shoot ethereal troops such as vampire count's spirit hosts or indeed plow through them in melee combat.
The +1 to hit can be particularly useful for models or units with multiple shots, empire volley guns and outriders and dark elf shades are units that benefit greatly from this spell. Of course +1 to hit and armour piercing in combat is a benefit to any unit.
Glittering robe is a defensive spell and grants the target unit a scaly skin save of 5+, there is also a higher level casting that makes the spell affect every unit within a small area. This scaly skin save is useful as providing the target does not already have a scaly skin save (lizardmen for example) it adds to any existing save. This can give lightly or unarmoured troops a save against light missile fire or low strength enemies, it can also boost more heavily armoured troops save up to 2+ and 1+ making them extremely difficult to shift.
Conversely it also makes the target(s) more vulnerable if your enemy has the lore of metal themselves
Gehanna's golden hounds
This is a direct damgae spell that causes 1 model to take d6 hits that use the metalshifting lore attribute, it can also be used to single out characters (or champions) within units though these get a "look out sir" roll. Often this can be used to your advantage if you actually want to take out the actual unit rather then the character or champion. It is also slighting cheaper to cast then searing doom but has a more specialised role.
Be aware that there are two cheap magic items, the dragonhelm and the gem of fire available to all armies that grant a 2+ ward save against flaming attacks as well as several other similar magic items that are army specific. This means it can be less effective if used as a "sniper" type spell of course you may be hoping that the model makes all it "look out sir" rolls and therefore obliterates rank and file troops.
Transmutation of lead
This hex spell lowers the target units weapon skill, ballistic skill and armour save by 1. It also has an even higher casting version which greatly extends the range. It is also I consider the weakest spell in the lore of metal.
Overall this spell isn't great for the casting value, fair enough -1 ballistic skill is always -1 to hit but -1 weapon skill often makes little or no difference. A -1 to all hit rolls would have been a better effect for the spell. The armour save reduction is useful but can be obtained for a cheaper casting value with enchanted blades or plague of rust (which is also permanent). Still if you have nothing else to cast it can help swing a combat in your favour or you can use it to stack with plague of rust and enchanted blades.
This is a direct damage spell that affects every model in the target unit any single wound model is killed outright on a roll of 5+, any model with multiple wounds is killed on a 6. No saves of any kind can be taken against this. Also any enemy unit nearby must test for stupidity at the start of its next turn (make sure you remember this).
Due to the nature of the spell this is more effective against bigger units and hordes where it can cause more casualties. The smaller the unit the less effective it is likely to be. Bigger target units also mean that more enemy units are in range of the stupidity test.
Which is the best lore of metal spell?
The lore of metal is a powerful lore particularly against foes likely to have heavy armour such as the empire and warriors of chaos (or any other army/player that likes cavalry). It is also useful against monsters with a high scaly skin save particularly if they have regeneration (dark elf hydra's) or are flammable (Wood elf treemen).
The usefulness of the lore of metal varies greatly against different army's and different compositions, the bottom line is its great against foes with a decent armour save but lowers in effectiveness if not. It can also be a useful "insurance" if the army you are facing can field models such as steam tanks or treemen that are otherwise hard to shift. A level one wizard with the default spell searing doom can often cause these otherwise impervious models a lot of trouble.
The Empire special character Balthasar Gelt, the supreme patriach is the only character currently who gets loremaster of the lore of metal, although the high elf special character Teclis may select it as can Daemons of chaos heralds of Tzeentch.