I think a new mob should be added. It would be the same as a normal spider except it would be reskinned. Also if you are hurt by it you are poisoned and you take damage every few seconds. The only way to cure yourself is to make an antidote, maybe with slimeballs. The new spider could drop poison glands which you could put on swords or arrows. The poison weapons would poison the mob or player.
And they can climb even higher.
but i really like the poison weapons idea!