The testing with the Dark Magic was strange, at first it left Arthur confused but he the more he tested the more confused he felt. The test was quite simple, before successfully growing a plant, he would fuse a bit of Dark Magic into it and try to merge them and if that doesn't work, he would try to fuse them together after the plant grew.
The latter is the method he used with the last few attributes as the first one rarely works, even in case of the Heavenly Arrow, he has to add the attribute after the core of arrow is created.
The simply grown plant wriggled on top of Arthur's hand, seemingly happy to be born, it wasn't completely sentient yet it was not unsentient too. When the most important part occurred and the Dark Magic slowly entered the plant's small body, the wriggling became fiercer and surprisingly, it started emitting screeching noises that were not so pleasant to the ears.