Official Riftbound ruling

What happens when I play Temporal Breach targeting my Elder Dragon in base and pay Any RuneAny RuneAny RuneAny Rune for Dragon Roost while playing Elder Dragon? Will it be played to base or to Dragon Roost?

You can choose which of those two locations you want to play it to.

In general, there are not many cases where we have this kind of competing instruction: one effect that tells you to play a unit to one location, and one that tells you to play it to another. That being said, when they do arise with contradictory requirements, the precedent from other areas of the rules is that the player chooses one of those requirements and ignores all others.

Note that this only applies to mandatory positive requirements, ones that specify a location or method without also preventing the action from being performed to other locations or with other methods and that aren’t optional. For example, an effect like Perched Grimwyrm’s passive ability is a restriction on playing that unit to any location other than a battlefield you’ve conquered this turn. Rockfall Path’s passive ability is a restriction on playing units to Rockfall Path. If you’ve conquered only Rockfall Path this turn, you can’t play Perched Grimwyrm at all, because those two restrictions are contradictory.

In this case, the contradictory requirements don’t directly prohibit the playing of these units to other locations, so we can choose one of the contradictory requirements and ignore the rest.

Riot's question and answer, word for word, from theVendetta Rules FAQ and Clarifications

The cards

  • Temporal Breach
    Temporal Breach

    Spell

    Hidden (Hide now for Any RuneAny Rune to react with later for 0 Energy0 Energy.)

    Banish a unit, then its owner plays it to the same location, ignoring its cost.

  • Elder Dragon
    Elder Dragon

    Unit

    Any amount of your damage is enough to kill enemy units.

    When you play me, choose up to one enemy unit at each location. Deal 1 to them.

  • Dragon Roost
    Dragon Roost

    Battlefield

    Any player may pay Any RuneAny RuneAny RuneAny Rune as an additional cost to play a Dragon. If they do, they play it to this battlefield.