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.

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.

If Master of Cruelties attacks alone, another creature entering the battlefield attacking will have no effect on it. Master of Cruelties continues to be an attacking creature. The other attacking creature will assign combat damage normally, regardless of whether Master of Cruelties's last ability triggers.

Master of Cruelties's last ability won't trigger if it attacks a planeswalker.

For a player's life total to become 1, what actually happens is that the player loses (or in some rare cases, gains) the appropriate amount of life. For example, if the player's life total is 4 when the last ability resolves, it will cause that player to lose 3 life. Other effects that interact with life loss (or gain) will interact with this effect accordingly.

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.