Can you play a spell like Cleave when there are no legal targets on the board, just to gain benefits like Legion?

Ruling: No, you cannot play a spell without legal targets. Spells need legal targets to enter the chain.

Nuances:

• The rule "If all of a card's instructions are impossible, it is still played and resolved, but nothing happens" only applies to spells that have already started resolving
• A spell never gets added to the Chain if it cannot choose its targets