A Warrior may choose to master one or more forms of Mighty Deed of Arms at a cost to all others. For each Warrior level, the character may choose one strictly defined type of Deed, such as disarming attacks, trips, or parrying to increase his Armour Class. The Warrior may attempt this Deed with +1d up the dice chain on his Deed Die. However, for all other Deed attempts, his Deed Die is decreased by –1d on the dice chain.
Thus, David the Defender wants to be able to use his shield to protect another at 1st level. His normal Deed Die is 1d3, so his Deed Die to perform this manuever is 1d4, but he has a mere 1d2 for all other Deed attempts. The next level, his Deed Die would normally improve to 1d4, but instead is reduced to 1d3. His attempts to shield others, though, increase to 1d5. He can choose to have a second special Deed (parrying to defend himself) at 1d4, but that again decreases all Deeds but his signature Deeds to 1d2.
And so on.
Using this rule allows characters to distinguish themselves, so that two Warriors of the same level have different combat styles. It allows each Warrior to do one thing really well, but at a severe cost in versatility. In addition to this rule, Warriors are allowed (nay, encouraged!) to “Quest For It” in order to gain a bonus to specific types of fighting without accepting such drastic penalties for so doing.
NOTE: Any class may be allowed to make a Faustian bargain of this sort, becoming better at one particular thing at the cost of much, much more. If you want your Thief to be better at climbing by taking the same penalty to every other Thief skill, we can talk.