I didn't really want to just have another rant today after two yesterday so I'm going to talk a bit about Liandry's Torment and why it's the worst possible upgrade that could have been used for Haunting Guise.
First of all, Haunting Guise is one of my favourite items. It's the best item you can get as an early-game aggressive mage. The problem with the item was that it fell off pretty damn hard once people started getting a good amount of magic resistance.
So, now when you upgrade Haunting Guise to Liandry's Torment you get a nice bonus to stats and a shiny new passive. Unfortunately, there's no increase to the magic penetration, only ability power and health.
When you damage a target with magic damage, the passive deals 2% of the target's current health as magic damage over 3 seconds and doubles the damage if the enemy is slowed or snared.
Not bad, right? Overall pretty nice.
It's extremely powerful in the right situations and I'm going to give you the tools to maximize it.
Quite often I will see people build Liandry's Torment on bursty champions that would be better off with some raw ability power, rather than this passive.
I feel like I die a little on the inside when I see a Nidalee or Lux buy this item.
If you want to maximize this item's usage, you need to have it up on the enemy as much as humanly possible and Nidalee, Lux, etc. who are more about raw poke damage don't really need it.
Instead of giving you bad examples of the item's usage, I'll give you a few good ones.
The best possible champion to have this item is a Season 2 favourite that is still recognized as the signature champion of Alliance's Froggen.
Massive AoE magic damage that slows enemies so you get the full 4%/3sec applied to as many people as possible for as long as possible.
Next best champion?
Drop that ultimate and hit them with an upgraded Death Laser and watch them burn. If your Gravity Field isn't enough to slow them for the full effect, get a Rylai's Crystal Scepter.
I see so many people run Liandry's on Brand that he gets an honourable mention due to it's synergy with his passive. You'll need a Rylai's Crystal Scepter to get it's full potential but, hey, that's not an issue.
In the end, Liandry's is a great item but it's not because of it being an upgrade for Haunting Guise.
So, if you're looking to harass enemies a little easier as Nidalee or Lux, get a Void Staff long before you get Liandry's Torment. If you want the Haunting Guise, go right ahead, I would just recommend selling it later in the game rather than upgrading it.
However, if you're playing a champion that can consistently slow enemies and keep applying Liandry's Torment, go nuts.
Liandry's is surprisingly good on Dr. Mundo and Amumu(especially if you can get a Rylai's Crystal Scepter or get Blessing of the Lizard Elder).
TL;DR when buying Liandry's Torment.
Area of effect damage per second is good.
Long range poke/line nukes bad.
AoE DPS good