Don't forget about the shard of readiness.
Would a quinted Purple Nymph with Readiness be OP? A lot of things are OP with SoR, so there is no way to balance that no matter how high the ability cost is.
If you use this for creature control, then Owl's Eye does almost the same thing for only 
. Also, you can easily Rewind or Mutate this creature.
Let's forget about SoR because you're right, you can't control for that.
A quinted Purple Nymph will still die in 2-3 turns with a Fire Shield or Thorn Carapace. You would need an antimattered quinted purple nymph to guarantee it's survival (a 3 card combo).
On the other hand, a quinted blood shade... opponent has or plays a fire buckler. After 4 turns, it still has 5 hp (assuming you've done NOTHING with it), making it NOW as strong as a quinted elite otuygh. And EVERY time you consume a creature, you can survive another 2 turns of the buckler.
Let's think about other growth abilities against fire buckler:
Devour - +1 / +1 -> it has to eat EVERY turn for its hp to remain constant and it will never rise. This is not an easy thing to do which I'm sure you'd know if you played otuyghs against fire buckler.
Growth - +2 / +2 -> grows by 1 hp per turn, which seems pretty good, but remember - Playing Lava Destroyer post-Fire Shield is insta-killed (7 | 1) and Forest Spectre starts at 2 | 3, a very weak creature to begin with. NOT to mention that in both these cases, the growth ability is a different element from the summon element.
Drain - +3 / +3 -> grows by 2 hp per turn AND starts at 9 hp for BloodShade AND all using the same element.
Now take away fire buckler, and think how fast it grows. After one drain (14 quanta), it becomes 12 | 12, for nearly the same quanta as a light dragon (13 quanta for 12 | 12)... except you know, it also removed -3 | -3 from an enemy creature AND it has an ability while light dragon has nothing.
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And this is all not even considering it's completely massive amounts of damage. Any deck except mono-aether immortal creatures with phase shield (or a quanta denial deck) would be run over by it. I wouldn't even use the SoR. Fractaled devourers to build quanta, and I'd momentum + quint them instead. Completely unstoppable.
Assuming it eats every turn (quinted + SoR'd, or just fractaled devours would make that an easy job), it does a cumulative 99 damage in 6 turns and 225 damage in 10 turns.
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My personal suggestion would be to make all legendary creatures non-status changeable. That is, they cannot become immortal, momentumed, adrenalined. Even then, I think this card is crazy overpowered, but at least it wouldn't be unkillable.
