I'm seeing it quite often and it's seemingly a must build on some champions (looking at you AP teemo) and some people really insist on forcing the Rylai's/Liandry's (super expensive man) combo build.

But how good is it really? I understand building Haunting Guise. It's cost effective for what it is but Liandry's is fairly expensive for what it's stats so i'm guessing it's the passive that makes people like it so much.

But it's 5% of current HP magic damage before MR or 10% if movement impaired (non aoe).

Going to late game, unless you're a tank/bruiser, I figure maybe 2000hp? The passive, if you had 0 MR, would burn for 66 dmg once a second for 3 seconds if you were at full hp the entire time. That seems pitiful for how good the item is considered.

Apart from those champions it just has natural synergy on (e.g Cassiopeia, Teemo) I'd consider it a situational pick for most champions. I was just now pondering the effectiveness of building it on Akali (Twilight Shroud is a slow, remember) but remembered that Akali is a burst champion, not a DPS mage. The Torment would be best reserved on her for facing champions she cannot burst down- specifically HP stackers like Mundo, Cho'Gath, or Sejuani. Getting it to burst squishy targets is completely wasteful. I'd say this applies to any AP champ who relies on burst for kills rather than DPS; I wouldn't buy it on Annie, Kassadin, Veigar or LeBlanc unless facing something seriously unburstable. It really depends on how well Deathfire Grasp (the other anti-HP-stacking mage item) works for you.

Liandry caters to a very specific subset of champions. These champions excel at whittling down their opponents from afar before the teams engage for a fight. They are characterized by low cooldown long range "poke" spells, or with trap abilities left waiting in common paths.

Teemo, for example, loves Liandry. Toxic Shot is already a DoT effect. When combined with his mushrooms alone he gets a huge damage increase in the fiend. When he gets a Rylai for some survival, he gets an even bigger damage increase, while also gaining increased damage off of Blinding Dart.

Depending on your team comp, Nid is a good candidate for Liandry as well, for very different reasons, as a situational item choice. Javelin Toss has a really low cooldown and really long range. Bushwack traps are single target, and have a fairly debilitating % Reduction debuff, while revealing you for more Javelins. Literally the only thing missing from this combination is Rylai to make both of these whittling abilities slow for double the Liandry damage.

Riot thought about this specifically while designing Liandry. It made even more sense when they built Liandry on Sejuani while testing it.

In most cases it's just people buying haunting guise for the early advantage knowing that they'll at least have something to upgrade it to later.
With evelynn I usually rush haunting guise then don't upgrade it until after i've finished my other core items (dfg/deathcap/void staff). The passive is nice even though it's not the most noticeable effect in the world.

It is usually better to stop at haunting guise.

I find it more useful in my builds as an upgrade to Haunting Guise than for its own passive. More Health and AP, while keeping the wonderful MPen in the case that the opponents don't have enough to make Void Staff worthwhile.

It's not bad, but not all champs can really use it.

It gives more damage, the dot, and a bit more health. Great for Poker's, like Kennen, and Beefy Ap like Rumble/Cho wo can use the health. Also, with Rylai's, it always does 10%,

I think the dots are negligible

Apologies for chopping up your long post, it was hugely insightful, thanks for that!

I'm curious about your own opinions on the actual planning of the Rylai's/Liandry's synergy because of how expensive that specific build actually is before it actually finishes to do anything. Not sure if you read the "pro" blogs or theorycraft blogs like Reign of Gaming, as it has Rylai's/Liandry's as one of the potential fail builds because of how long it actually takes for the specific build to get to that point. Since it's an AP specific build, you can't really rush it unless you heavily delay Deathcap so your power is much lower than it should be comparatively unless the argument is that Liandry's is rarely actually built on champions that would build Deathcap in the first place.

Liandry's is fantastic for champions like Xerath or Lux, long-range pokers who start the fight with harass. Since the passive is on %Current HP, it's noticeably less effective on other champions, but it's still nice for the HP, AP and MPen. It's a very well balanced way to give more power to poke champs without buffing all casters.