Oathsworn Knight
Creature — Human Knight
This creature enters with four +1/+1 counters on it.
This creature attacks each combat if able.
If damage would be dealt to this creature while it has a +1/+1 counter on it, prevent that damage and remove a +1/+1 counter from it.
Illustrated by
Throne of Eldraine#354RareEnglishLegality
Rulings
If multiple sources would deal damage to Oathsworn Knight simultaneously (for example, multiple creatures blocking it), its last ability applies once. All of that damage is prevented, and only one counter is removed.
Oathsworn Knight's last ability removes only one counter from it each time it would be dealt damage, not one counter per 1 damage prevented.
If Oathsworn Knight can't attack for any reason (such as being tapped or having come under that player's control that turn), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.
If damage that can't be prevented is dealt to Oathsworn Knight, one counter is still removed from it.