Thanks everyone for so much feedback.
Some details:
The group was resting at camp during late morning and had left their horses grazing near some trees and they knew there was forest very near by. They noticed three humanoids accompanied by four spiders taking their horses away. When the PCs come to stop them, the three humanoids run and the spiders stand their ground to allow for escape. The spiders paired off and 5 characters engage the spiders while the three humanoids run toward the forest with the horses. The humanoids have about two rounds of a lead on the PC who decides to give chase. He uses land strider to catch up with them and does so within another three rounds. He sleeps the horses, so the ettercaps turn on him.
There was discussion at the table that these were likely ettercaps. I should have shown a picture, perhaps? But confirmed they were ettercaps when the horses dropped.
This party... Doesn't have a lot of commeraderie. The players all get along well, and we have plenty of laughs around the table. But, due to some role play choices, there are definitely some tensions in the group (I have tried to provide a variety of ways for the group to bond, but, thus far, to no avail). So, once the threat of the spiders was gone, one PC decided to simply return to camp, but the others went... I think more to save the horses.
So, I did not provide warning that this might be a fatal choice for the PC to give chase. He asked about the terrain, which I said was lightly treed, and not yet fully forested. He did not ask about the humanoids, and I did not offer up any info either - which is where my fault lays.
The other part that I got wrong was the poison effect, which he should have saved against each round, but I also did not give him damage from the poison, only the disadvantage to attacks and saves.
My last mistake was asking him to do his death saves at disadvantage because I had written something down wrong in my notes, and didn't realize that it only applied to ability scores. So, what is in question is only his last death save. The others would have been fails regardless of the disadvantage. All my mistakes

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