The spider is a very strong event, it is substantially 3 trivia questions about the game, make one mistake and you loose some hp (I don't loose the trivia game in so long), if you make all three questions correct you are rewarded with a basic random & random die. This is basically a die that works as both its parts at the same time (terrain and mirror aren't possible parts of these dices), basically a "block & heal" die which rolls a 4 heals you for four and blocks for four.
All of these dices are waaaaaay stronger than the rest, to the point that they are a must for floors 40 and higher.
These are what are known as "half-dice" because they are half something and half something else