The mechanics of Mirrodin Besieged have been revealed. Now that Mirrodin is at war in full, they are pulling out the stops, and Phyrexia is ready with answers around every corner. From being painfully unaware of the threat that was Phyrexia to declaring war on them, the Mirrans are up for the fight.
There are a few mechanics that are returning from Scars of Mirrodin: Metalcraft, Infect, Proliferate.

Along with these, there are three new mechanics. First off, there is Battle Cry.
A creature with Battle Cry gives each other attacking creature +1/+0 until end of turn if it attacks. Multiples of these stack, but just remember that they don’t power themselves.

Next, “poisoned”. This isn’t an ability per say, but it is a state that can now be monitored. Basically, it checks whether or not you have a poison counter. For example, Septic Rats get +1/+1 until end of turn when it attacks a player that is poisoned.
Lastly, the Phyrexian ability, Living Weapon. Living Weapon works like this. If it enters the battlefield, it puts a 0/0 Germ creature token on the battlefield. Now, normally, this token would go directly to the graveyard, but in this case, the equipment attaches itself to that token. Wizards say that each Living Weapon equipment will give at least 1 toughness, so your germ tokens won’t go to the graveyard.

It’s hard to say how these new mechanics will work without seeing in what context they will be put. After a few more cards are added to the visual spoiler, we might get a better idea. Hm.