At the time of Onslaught, Walls and Legends were still creature types with special rules attached; "Legend" wasn't a supertype yet and the Defender keyword wasn't invented yet. This led to acrobatic "non-wall, non-legend" clauses on many Onslaught-block cards. See Mistform Ultimus, and compare the Time Spiral printing to the Legions printing.
That meant that this could be used to turn opposing critters into walls so that they couldn't attack you. Now it's much less useful.
Posted By:
BegleOne
(7/16/2011 9:33:09 AM)
@Kryptnyt Unnatural Selection is a good point.
Keep in mind though that back then you could choose Wall and Legend, which both came with special rulings.
So this card once allowed you to cast it on an opponent's creature to prevent it from attacking by making it a wall, or - an admittedly very rare case with a second Mistform Mask - allowed you to destroy two copies of one creature by making them both legends.
I hope when Wizards one day decides to print something like this they'll make it a cantrip for 
.
Posted By:
Mode
(5/1/2012 1:04:34 AM)
But... Unnatural Selection.
Posted By:
Kryptnyt
(4/1/2010 12:12:43 AM)
Familiarity, the first myth of reality: What you know the best, you observe the least. Mistform Mutant
Devotion, the second myth of reality: The faithful are most hurt by the objects of their faith. Mistform Dreamer
Conviction, the third myth of reality: Only those who seek the truth can be deceived. Mistform Skyreaver
Fellowship, the fourth myth of reality: As the tides of war shift, so do loyalties. Mistform Wall
Trust, the fifth myth of reality: Every truth holds the seed of betrayal. Mistform Mask
Posted By:
BloodDragon
(3/5/2013 10:14:56 AM)
note that this doesn't mention anything about not being able to chose wall 5/5 also works for taking down/building up things whose p/t work with creature types, and goatnapper i think this is a pretty damn good blue card
Posted By:
ChampionofSquee
(3/24/2010 10:58:58 AM)
looks usable somehow
Posted By:
Imperialstonedragon
(3/24/2011 2:48:57 PM)
@ChampionOfSquee- You can choose wall but it won't give the creature defender so won't really help.
There is much better stuff for doing this effect around.
Posted By:
Studoku
(5/10/2011 3:09:27 PM)