Master of Cruelties

Assigning no combat damage isn't the same as preventing that damage. Effects that make damage unpreventable will have no effect if no combat damage can be assigned.

Master of Cruelties isn't forced to attack, but if it does, it must do so alone. If you control another creature with an ability that says it must attack if able, that creature must attack and Master of Cruelties won't be able to.

An ability that triggers when something "attacks and isn't blocked" triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.