http://forum.square-enix.com/ffxi/threads/19515-dev1065-Relic-Mythic-and-Empyrean-Weapon-Upgrades?p=261278#post261278
CLIFFNOTES:
-Legendaries are upgraded in two stages through ToM
-Stage 1: upgrades may include damage, delay, attribute and weapon skill damage
-Stage 2: upgrades will include special animations along with "Afterglow" sphere effects that grant status benefits to party members (SPHERE EFFECTS ARE EXCLUSIVE OF WIELDER)
-First stage Magian trials allow a majority of players to complete them. Second stage Magian trials will be exceedingly difficult to complete and only a select number of PCs will be able to upgrade weapons to this stage.
FULLY UPGRADED MANDAU:
DMG:55 / Delay:176 / Attack+40
"Mercy Stroke" +40%
Additional effect: Poison
Increased chance of dealing extra damage
Afterglow Increases Accuracy and Ranged Accuracy
FULLY UPGRADED TWASHTAR:
DMG:55 / Delay:176 / DEX+20
"Rudra's Storm"
Aftermath: Occ. deals double damage
Afterglow Increases critical hit rate