Urborg
Legendary Land
: Add 
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: Target creature loses first strike or swampwalk until end of turn.
Not even an eagle poised against the sun would have marked the hobbits sitting there, under the weight of doom, silent, not moving, shrouded in their thin grey cloaks.
Greg Hildebrandt & Tim Hildebrandt
Rulings
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.
(wotc • 2009-10-01)
Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.
(wotc • 2009-10-01)
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.
(wotc • 2009-10-01)
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.
(wotc • 2008-04-01)
The ability can be used on a creature without First Strike or Swampwalk but has no effect.
(wotc • 2004-10-04)
Rulings
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.
(wotc • 2009-10-01)
Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.
(wotc • 2009-10-01)
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.
(wotc • 2009-10-01)
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.
(wotc • 2008-04-01)
The ability can be used on a creature without First Strike or Swampwalk but has no effect.
(wotc • 2004-10-04)