This + Double Radiant Elemental + Double Shadow Vision into Double Vivid Nightmare + Embrace the Shadow + Vivid Nightmare on Reckless Experimenter + Double Corrupt Healbot + Vivid Nightmare on Corrupt Healbot = 24 damage. This card is broken AF Kappa
You don't have the mana for that.... It only discounts deathrattle minions. Read the card text clearly. It's not: "Deathrattle: Minions" but it's "Deathrattle minions". The effect is an "aura" effect that lasts as long as the minion is on the board; It's not a trigger on deathrattle effect. Vivid Nightmare would cost 1 in your scenario and after Reckless Experimenter and 2 Radiant elementals you can only play it once.
This is going to blow up wild meta and possibly standard. Play this with deathlords and cubes for some disruption and sylvanas. You play deathlord sylvanas then cube either sylvannas or deathlord depending on your position. Mark my words this is meta defining.
It's not. I read this card wrong the first time as I suspect you did as well. It is not triggering a deathrattle effect (bolded Deathrattle got me). It is saying DEATHRATTLE MINIONS you play cost 3 less. So the card is in effect only as long as this is on the board, and it only affects deathrattle minions that you play during that time.