Lineprancers

When Lineprancers enters, you get {TK}{TK}, then you may put a power and toughness sticker on a creature you own.
{3}{G}: Target creature you don't control blocks target creature you control with a power and toughness sticker on it other than Lineprancers this turn if able.

Unfinity

Activating the last ability doesn’t mean you have to attack with the target creature you control.

Creatures other than the first target creature may also block the second target creature.

If a creature is required to block a creature with menace, another creature must also block that creature if able. If none can, the creature that’s required to block can block another creature or not block at all.

If either target of the activated ability is illegal as the ability tries to resolve, it will have no effect. If both targets become illegal, the ability won’t resolve at all.

If the creature you control does attack, and the creature you don’t control can’t block it for any reason (such as being tapped), then it doesn’t block. If there’s a cost associated with having it block the creature, its controller isn’t forced to pay that cost, so it doesn’t have to block in that case either. That creature is then free to block another creature or to not block at all.

Once the activated ability resolves, the blocking requirement is set, even if the second target creature loses its power and toughness sticker before blockers are declared.