1. How do I quantify the damage? This shouldn't take damage from multiple sources unless it doesn't die because the damage wasn't enough (i.e. guikd is buffed to a 3/3, it could take 2 damage from a source, then 1 or more from another single source).

If this could mechanically function, should it appear in multiple instances to the point it becomes a mechanic?

Okay, okay, slow down. To find out what the wording does, let's see how you want it to work.

Let's say your opponent attacks with two 7/7 tramplers and each of them is blocked by two 3/3s. How is damage asignment supposed to work?

Damage from one trampler could be dealt to this creature instead. This creature does because, assuming this creature isn't buffed, this creature has one toughness and is being dealt seven damage. Six of that seven damage will go through to you, like normal trample, because this creature's toughness is one.

Wait, you said each of the 7/7s is blocked by two 3/3s. In that case, after the controller of the attacking creatures has assigned the order in which your blockers are dealt damage, you may choose this creature to take the damage that would be dealt to one of those blockers. So, assuming no buff occur, the 7/7 deals 3 damage to one 3/3 blocker (blocker A for reference) and 4 to the other 3/3 blocker (Blocker B for reference) , you may choose to have this creature take the 4 damage that would be dealt to Blocker B. Blocker A dies because 3 damage will kill a 3/3 and this creature dies because 4 damage will kill a 2/1. Applying trample, 3 damage will go through to you. If this creature took the 3 damage for Blocker A, then a total of 3 damage would go through to you from trample, 2 from trampling this creature that was dealt 3 damage, and 1 from Blocker B who was dealt 4 damage.

Okay, hypothetical number two. What if you have 2 of these in play? Where does the damage go?

If two copies of this were to redirect damage from a single target to them, the damage would be divided by the player controlling the source assigning damage as normal.

To continue using the trample example, a 7/7 trample's damage is redirected to two copies of this creature. If no buffs occurred, that's 7 damage split between two copies of this one toughness creature. If I understand trample correctly, 5 damage should go through to the defending player, since five should be the excess damage after the combined total 2 toughness of these two 1 toughness creatures.

If two of copies of this creature redirected damage from a single creature that had a power of 1/3 to them, then one of those copies would not receive damage? If two creatures block a single source, but only damage is being dealt, that 1 damage can't be split to any lower number, right?

> If I understand trample correctly

I think it's likely from above explanations that you don't understand trample correctly (at least its nuances): Excess damage from a creature with trample doesn't automatically "go through" - the controller of the source has a say in it.

So one thing I notice that I really didn't expect before and that becomes actually relevant in other examples is... you want this to redirect all damage from an untapped creature to the creature with allegiance at once?

Example: If a 3/4 with deathtouch blocks a 2/4 with vigilance you control while you control two of these... how many of your creatures are going to die?

Yes, allegiance is intended to divert the total amount of damage from a single source from an untapped creature to the creature with allegiance.

I just now realized that my current wording makes allegiance mandatory and also doesn't specify that the untapped creature must be yours. Under the current wording, both creatures with allegiance would die. Neither the 3/4 deathtouch nor the 2/4 vigilance creature would die. Since the current wording is mandatory, damage from the blocking deathtouch creature would be dealt to both of these creatures. The defending player could choose to have all 3 damage from the blocking deathtouch creature go to one copy of this allegiance creature, while the other copy of the allegiance creature receives no damage, in which case only the allegiance creature dealt damage would die. Since all damage the 3/4 deathtouch creature would deal is diverted away from the 2/4 vigilance creature, the 2/4 vigilance creature would receive no damage from the deathtouch creature, so the 2/4 vigilance creature will not die due to damage from a creature with deathtouch.

In order to make the effection optional instead of mandatory and remove the drawback of having to have a player's allegiance creature receive damage that would be dealt to an opponent's creature, I would like to change the wording to "You may redirect damage that would be dealt to an untapped creature you control to this creature instead."

If you believe this ability should target, then I would change the proposed wording to "You may redirect damage that would be dealt to target untapped creature you control to this creature instead."

The order of replacement effects matters, so each source of damage is redirected to a single creature with allegiance.

I think it's worth noting that there's a reason Wizards decided to write Shaman en-Kor the way it is written. But I also recognize that there are problems with that implementation too, so I couldn't begin to pretend to know what the 'best way' to do an ability like this is.

If the ability has appeared once before at rare, then it's probably not feasible to do at common. This idea is scrapped.