Mauramana is quite underrated
I've started to play around with this in my build and I must say it's a sleeper item. It has a god-awful start where it takes a morgana snare duration to finally charge up to 750 mana, but it's quite worth it if you can delay the game that long. The only downside is you have to have your build revolving on this item to benefit but I will compare it to a mainstream build in terms of DPS.
Champion - Kog'maw Stats at level 18: AD - 98.43 Mana - 1002 0.964 (0.665 base)
His highest % winrate build is
Stat on this build will be
2.35 AS
296 AD
74 AP
His DPS with W active, AA alone, average crit damage on a 0 resistance target will be
905 AD damage + 53 AP damage + 28% of target current health in AD + 25% of target max health in AP
Factoring his burst damage by landing his Q E R spells he would deal an 923 extra damage
His build around Muramana will be
Stats on this build
AS - 2.35
Mana - 2652
AD - 294
AP - 174
His DPS with W active, AA alone on a 0 resistance target, and 2600 mana because I'm to lazy to factor in mana reduction will be
844 AD damage + 53 AP damage + 28% of target current heath in AD + 28% of target max health in AP + 131 from average spell blade Proc
Factoring his burst damage by landing his Q E R spells he would deal an 1109 extra damage
That said, not every champion can benefit from Muramana but there's a decent amount of champions that would scale well with it into late game despite it not in their popular build.

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