If I conquer with Lucian and I play a card after, but then I remember I forgot to ready Lucian after conquering, can I still ready him?

No, you cannot ready Lucian after you have already played another card.

According to RiftJudge FAQ #8229, Lucian's ready ability is a triggered ability that occurs when he conquers. Because it is a triggered ability, it must be placed on the chain at the moment the conquer occurs.

If you missed the trigger or failed to resolve it when it was on the chain, you cannot go back and "retroactively" resolve it after performing subsequent actions (like playing another card). Once the chain has resolved and you have moved on to playing another card, the window to resolve that specific trigger has closed.

Key points:

  • Trigger Timing: Triggered abilities must be addressed when their condition is met.
  • Chain Resolution: Once you play another card, you have effectively passed priority and allowed the previous chain (where the conquer trigger existed) to empty and resolve.
  • No "Save for Later": As noted in the FAQ, you must use the ability when it triggers; you cannot save it for a later time in the turn.