The Grail Quest

In a campaign I'm doing that's based on the table top version there is an optional rule that we use, the effects are listed below. My friends Leader already has 4 toughness, does this put him over the human maximum because it is a magical effect?

The Grail Quest
Bretonnian warbands may replace any 6 of a kind rolled for exploration with the following;

The Questing knight has proven his worth, in the distance, he spies the lady beckoning him, his compatriots see nothing but know exactly what is happening and know he must go alone. As he approaches the lady she extends her hands, saying nothing, but within them is the Grail, in all its understated glory.
The knight drinks deep and is forever changed. All the Questing knights’ permanent injuries are healed, his title is changed to grail knight and he gains the Blessed special rule.

Blessed: The Questing knight gains +1W, +1T., in addition, the Knight’s and his mount’s attacks are +1 to hit in close combat and in addition do double wounds to Possessed, Daemon and Undead models

That is a very very powerful (imho too powerful) boost

Although Human Toughness cannot be increased above 4 (as per core rules) items like Mandrake can give it a temporary boost over the maximum, but at a serious potential risk. So potentially yes, the knights T would go above 4.
But I am not convinced a Human with Toughness equal to a Rat Ogre is a good thing in the game.

Your blessing lacks any risk to balance it out and certainly should not be a permanent boost.

I agree this is over powered. Meaning, it changes the game mechanics for a specific warband. I do like the concept. But off the top of head, I would suggest something different:

I would the change: The Leading Questing Knight can seek The Lady of the Lake for her blessing instead of exploring for wyrdstone or any other post-game events. Make a random roll. Perhaps, a leadership check to see if she’ll even have audience with the Questing Knight to plead his case. Then you can have a chart for the gift. Perhaps a potion that temporally raises a Character Stat 1d3 game, 1d3 Healing Herbs, 1d3 Tears of Shallaya, or a Lucky Charm, ect. Make a descending chart form good gifts down to something weak or nothing. But only one gift per visit. Giving the chance the Lady of the Lake may not take audience with the Questing Knight, helps keep some balance and add some flavor to the game.

The Gift chance to the Knight, is a bonus that is offered to the Knights and not every Band at the table is offered. So, it should have a penalty. Meaning, that it should cost something. In this case, The Questing Knight doesn’t participate in post-game activities, since he has the chance to receive gifts without purchase, searching, ect. But still gains XP and leveling as normal.

This is just a suggestion for ideas, and its your gaming group.

From the sounds of it, completing the Grail Quest sounds like a "You Win!" event. (Cue animated fireworks & game credits)

I'd be inclined to say the Grail Knight can go over normal human maximums but either leaves the warband (perhaps joins the Dramatis Personae for the campaign setting) OR only appears if the warband is an underdog (like Bertha!). I mean, he's a Grail Knight, he shouldn't be scrabbling about on treasure hunts or gang fights.