Urborg

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 — October 01, 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.

Wotc — October 01, 2009

Urborg’s second ability affects only the first strike ability, not the part of the double strike ability that acts like first strike.

Wotc — October 01, 2009

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 — April 01, 2008

The ability can be used on a creature without First Strike or Swampwalk but has no effect.

Wotc — October 04, 2004