When rule 421.2.b applies (movement restrictions causing recall), does this prevent abilities like Force of Fortune from being added to the chain to move a unit from Vilemaw's Lair to base, or does the unit get recalled instead?

Ruling: Rule 421.2.b applies to movements that become illegal after the ability is already activated/added to the chain, not to movements that are illegal at the time of targeting. Abilities like Force of Fortune cannot be added to the chain in the first place if the destination is illegal (like moving to base from Vilemaw's Lair, or moving to a battlefield with two other players' units).

Sequence:

• When adding an ability to the chain, check if the destination is a valid location
• If the destination is not valid at targeting time, the ability cannot be added to the chain
• Rule 421.2.b only applies when a movement becomes illegal during resolution (after it was already validly added to the chain)
• If a movement becomes illegal during resolution, the unit is recalled instead

Nuances:

• An example scenario where 421.2.b would apply: In a 3-player FFA using Promise Future, if multiple players use Ride the Wind to move to the same open battlefield at reaction speed, the first two units move successfully but the third unit's movement becomes illegal and is recalled
• There is ongoing debate about whether you can target illegal destinations (similar to targeting already-buffed units with buffs), but the prevailing interpretation is that illegal destinations cannot be targeted when adding abilities to the chain