If multiple attacking creatures must be blocked if able, the defending player must assign at least one blocker to each of them if possible. For example, if two such creatures were attacking and there were two potential blockers, they couldn't both be assigned to block the same attacker.
The constellation ability doesn't force any specific creature to block the target creature.
An Aura spell without bestow that has an illegal target when it tries to resolve won't resolve and will be put into its owner's graveyard. It won't enter the battlefield and constellation abilities won't trigger. An Aura spell with bestow won't be countered this way. It will revert to being an enchantment creature and resolve, entering the battlefield and triggering constellation abilities.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Jack Wang
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The defending player must assign at least one blocker to the target creature during the declare blockers step if that player controls any creatures that could block it.
A constellation ability triggers whenever an enchantment enters the battlefield under your control for any reason. Enchantments with other card types, such as enchantment creatures, will also cause constellation abilities to trigger.
When an enchantment enters the battlefield under your control, each constellation ability of permanents you control will trigger. You can put these abilities on the stack in any order. The last ability you put on the stack will be the first one that resolves.
