Oracle Text

T: Add B.

T: Target creature loses first strike or swampwalk until end of turn.

Card Rulings

10/4/2004 The ability can be used on a creature without First Strike or Swampwalk but has no effect.
4/1/2008 If a creature loses first strike after first strike combat damage has been assigned, it won’t deal damage during the second combat damage step.
10/1/2009 You choose whether the targeted creature loses first strike or swampwalk as the ability resolves. If the creature has both abilities, it will lose just one of them.
10/1/2009 Causing a creature to lose swampwalk is useful only during the declare attackers step or earlier. Once the declare blockers step begins, it’s too late to block the creature.
10/1/2009 Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.