If my Rift Herald dies, am I obligated to play a unit from my hand or can I choose not to?

No, you are not obligated to play a unit.

Rift Herald's Deathknell says "Play a unit from your hand" — "a unit" specifies a type/quality of card, and your hand is a Private zone. Per the official 2026-04-29 rules clarification (which takes precedence over the general "mandatory unless 'may'" principle), a player cannot be compelled to perform an action on cards in a private/secret zone when that action specifies a type or quality of card. If such an effect would compel you, you may ignore it instead.

The clarification explicitly cites Rift Herald as the exact scenario this rule was created for. So if you don't want to — or can't afford the Power cost — you simply say so, and the Deathknell does nothing.