The chair was the first to hit Black Knife. As a rank 2 giantess, she has the rank 1 Strength Boost divine ability, so her strength is comparable to a rank 3 godclad. All of that strength made the chair break immediately; it smashed into Black Knife.
The attack was nowhere near enough to kill Black Knife. He was at most bruised by the hit. The critical thing was that the blow forced him to stumble, so he couldn't dodge the next batch of seeds thrown at him.
The seeds were already glowing before they hit him. When they did, they began growing roots immediately.
If Black Knife distanced himself, he would be able to have the time and space to resolve this problem. But he didn't do so. He still kept stabbing Silver Mirror.
The seeds grew roots that stabbed into his clothes and into his flesh. Then the seeds began to drain him of his life energy.
