Sunset Paragon is correct.
Half of a decade is what you must pay,
It is much for what I am but not for what betrays,
Costs 5 mana (half of 10 ten years is 5), the cost is high for its bad body but the effect (the "betrayal" of enemies dealing damage to themselves) is powerful
The count of charm is what I deal,
Just not to those who gather near,
3 times the charm (3 is its attack), but it doesn't deal 3 damage to the enemies around it when summoned, It deals their attack instead.
Sadly just a pair is all I can I take,
It matters not for my effect is still great
It can only take 2 damage (a pair = 2), it has 2 health. But it's ok because you only really play it for its effect, which is great.
Night Fiend fits REALLY well for this one, though I did have Sunset Paragon in mind.