Istus is pivotal not only to the destiny of heroes as her portfolio and the module Fate of Istus would suggest, but she is also a mean combatant according to the 83 Boxed Set.
Back in the days of AD&D, it was entirely possible for PCs to take on gods. The 1E Deities & Demigods is loaded with stats and rules on the powers of these beings. This template carried over to the early Greyhawk products and articles thus giving the DM an option if divine wrath ever needed to be carried out. These gods were damn tough but not impossible to kill. Hit points capped out at 400, AC never went higher than -10 (AC 30 by today's rules, thanks Matrox*) and most had very high magic resistances. Let's examine how ridiculous it is to tempt fate herself in combat.
Istus resides on her own pocket plane most of the time, but when on the Prime Material Plane she assumes various forms from ancient crone to noble lady to simple shepherd girl. While amongst mortals, Istus never carries an obvious weapon, though her spindle, from which she creates her webs of fate is her true weapon. Despite this, any object she uses always hits; she controls fate after all. So yes, she could kill you with a cloak pin. Also, forget trying to sneak up on this little girl, she is the goddess of future events, she already knows whether you're attacking or not.
Provided you are still persistent enough to push Istus, her spindle has the following powers (insert your own rules).
Strand of Binding: You're trapped in iron chains. Istus is being nice.
Strand of Cancellation: Your favorite magic item? Never existed. Even artifacts are only partially affected.
Strand of Death: I like to think of this one as rapidly aging your character 1,000,000 years instantly.
Strand of Hostility: Suddenly its your destiny to protect Istus, even against your friends. Yikes.
Strand of Passage: Pesky hero, let's send you away to the Plane of Shadow, or the Ethereal Plane, or heck, Faerun.
Strand of Sending: She's feeling nice and just sends you several years into the future.
Web of Enmeshment: Whoa you're trapped in a maze of your own mind!
Web of Entropy: Great! Now she's just drained all your magic and spells in this area.
Web of Stars: This one transports everyone including Istus to her bizarre homeplane of webs and starry portals. Good luck finding your way out unless you are a genius level intellect.
So Istus basically toys with opponents or outright slays them. Even if you got a few hits in, she has 377 hit points (a ton in 1E terms) and is only harmed by +4 weapons. Even if you loaded up with big blades like the Sword of Kas and Blackrazor, she can once per day move into the future and back to heal herself. Congratulations for making it this far foolhardy adventurer. It is lastly mentioned she is served by a strange companion, a time elemental prince. As far as I know this type of creature doesn't exist in any other D&D rules, so I expect it can do all the above and then some. (*and now I know they do, thanks ProfArmitage, see comments below) Perhaps your PCs should pick on someone else easier, say Hextor or Erythnul.
Until next time...