Master of Cruelties

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.

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.