Is Lyandry's torment better than void staff for fiddle's ult?

If they're stacking MR, you're going to want void staff. Liandry's eats through people stacking health.

that's situational. if the team is all squishy then yes you probably want the torment. Flat Magic pen hurts squishies bad more then it would say a tank. If the team has a tank/tanks you probably want the void staff for the % magic pen it gives.

how much natural magic resist do tanks have? and also isn't liandry way better most of the time, i rarely see more than 2 mr items on tanks and usually there are only 1 or 2 of them each match.

Everyone if i recall right starts out with 30 mr...could be wrong. Also some champions get MR a level and some don't

how can i check someone's mr? and how much mr do they need to have to make void staff better?

i used it and it's not really good. i'd say void staff seems to be better lol

Flat Pen is better for ranged champions + a select few melees that do not get MR per level and choose to not build magic resist. The more magic resist a target has, the less it does for you since resistance gets less efficient the more you have.

Out side of that Torment is mainly good against health stackers.

No. However, if the other team doesn't build MR at all, Haunting Guise is better than Void Staff because it's cheaper and makes you slightly tankier.

The problem with Liandry's is that the %hp damage is very underwhelming, but you also end up paying a lot for it.