After I held this round with Last Rites, can I play a unit at any time during the round with its effect or only during the beginning phase when hold is resolved?

Based on the card text and the rules of Riftbound, here is the ruling regarding when you can use the effect of Last Rites:

Ruling

You can only use the effect of Last Rites at the specific times the trigger condition is met.

According to the card text, the effect triggers "When I conquer or hold." This means the ability is placed on the chain only when you perform a "Conquer" action or when you perform a "Hold" action (which occurs during the Beginning Phase). You cannot play a unit from your trash using this effect at any other time during the round.

How it works

1. Trigger: When you perform a "Hold" action during your Beginning Phase (or a "Conquer" action), the Last Rites ability triggers and is placed on the chain as a pending item.
2. Resolution: When that trigger resolves, you are given the opportunity to play a unit from your trash.
3. Timing: Because this is a triggered ability, you must resolve the "play a unit" instruction during the resolution of that specific trigger. You cannot "save" the trigger to use later in the turn.

Important Clarification

• Not a Passive Ability: Last Rites does not grant you a continuous ability to play units from your trash at any time. It is a triggered ability that creates a specific window of opportunity tied to the "Conquer" or "Hold" events.
• Costs: Remember that even when playing a unit from your trash via this effect, you must still pay the unit's energy cost (as noted on the card: "You still pay its costs").

This is my interpretation based on the card text and Rule 383.4.d (Hold Effects). No official FAQ exists for this specific timing question, but it follows the standard mechanics for triggered abilities.