I know the Book of Knights list them. Or one version of them and give a sentence to their power, but in games rules how would you translate those powers to make HARD rules? ( like what bonoses, etc...)

Dyrnwyn, gleddyf Rhydderch Hael: White-Hilt, the Sword of Rhydderch Hael: if a well-born man drew it himself, it burst into flame from its hilt to its tip.
(Magic sword which bursts into flames, should the sword be used for an unjust act the users is consume by the fire. Bonus to hit/dmg)

Mwys Gwyddno Garanir: The Hamper of Gwyddno Long-Shank: food for one man would be put in it, and when it was opened, food for a hundred men would be found in it.
(Says it all above, useful for feeding armies and starving peasents alike)

Corn Br?n Galed o'r Gogledd: The Horn of Bran the Niggard from the North: whatever drink might be wished for was found in it.
(What more can you say, have a good nights drinking.)

Car Morgan Mwynfawr: The Chariot of Morgan the Wealthy: if a man went in it, he might wish to be wherever he would, and he would be there quickly. Also known as the Chariot of Arianrod.
(Reduces travel time, half, quarter or instant)

Cebystr Clydno Eiddin: The Halter of Clydno Eiddyn, which was fixed to a staple at the foot of his bed: whatever horse he might wish for, he would find in the halter.
(After a wish and a goodnights sleep, find horse of any type in the halter).

Cyllell Llawfrodedd Farchog: The Knife of Llawfrodedd the Horseman, which would serve for twenty-four men to eat at table.
(When used to carve a joint of any size no matter how small, it will provide meat for 24 men.)

Pair Dyrnwch Gawr: The Cauldron of Dyrnwch the Giant: if meat for a coward were put in it to boil, it would never boil; but if meat for a brave man were put in it, it would boil quickly (and thus the brave could be distinguished from the cowardly).
(The pot is filled with a the ingredieants of a stew, less the meat, the man should then put his meat in the pot and make a valorous/cowardly check if he passes the pot instantly begins to boil, else the cowards meat just floats in the cold stew)

Hogalen Tudwal Tudclyd: The Whetstone of Tudwal Tudglyd: if a brave man sharpened his sword on it, if it (then) drew blood from a man he would die. If a cowardly man (sharpened his sword on it), he (his opponent) would be no worse.
(The man must make valorous/cowardly check, success grants a bonus to dmg/attack for the first attack, failure has no effect).

Pais Badarn Beisrydd: The Coat of Padarn Red-Coat: if a well-born man put it on, it would be the right size for him; if a churl, it would not go upon him.
(Magical chain armour that can only be worn by those of noble birth, grants extra protection)Gren a desgyl Rhygenydd Ysgolhaig:

The Crock and the Dish of Rhygenydd the Cleric: whatever food might be wished for in them, it would be found.
(As it says, a nice meal for one)

Gwyddbwyll Gwenddoleu ap Ceidio: The Chessboard of Gwenddolau son of Ceidio: if the pieces were set, they would play by themselves. The board was of gold, and the men of silver.
(As it says, but the player can also play the invisible opponent and grants a gaming check when used)

Llen Arthyr yng Nghernyw: The Mantle of Arthur in Cornwall: whoever was under it could not be seen, and he could see everyone.
(As it says invisibility)

Dude.... That was frickin awesome. Thank you. :o