Correct! (told ya it was easy :P), the answer is Odin/Dark Matter.
A black horse you can make me ride,
From the moment you reach the night. - You can buy Dark Matter once you reach Treno, where it is always night.
If you hope to get me at that point in possesion,
Work! I am not earn't in one training session. - The sommun Odin
costs alot of AP, but the Dark Matter also costs alot of gp's.
Only one time I can inflict great pain, - Dark Matter can be used once to inflict 9999 damage to any opponent.
But with a blade there's more to gain. - But if you equip it on "Dagger" she will learn the Odin summon.
The pointy use will always kill, - If Dagger uses her summon it will result in the Death status (thus killing).
But you need luck, or it will fail. - It works with chance percentages.
At first that use might seem too skilled, - Well, people may think it's overpowered.
No need to worry, chiefs won't be killed. - But Odin will never
instakill bosses.
I am no plane, not a Dino either, the guessing has begun,
No more hints I will give, doing that would ruin the fun.
Those two sentences are just to end the riddle.
I had to put in an anagram though (Dino).
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Quin, you're up!
Huh? I thought you got one for beating Ozma and one at Treno auction house?