Diadem of Songs needs a rework.
I saw a game designer mention that Actualizer was probably too niche, and that because it never really found its audience, it would eventually be reworked or removed from the game.
I think there is another item in a very similar situation: Diadem of Songs.
The item has a 0.6% pick rate, and it is basically only optimal on Seraphine. Even on Sona, where you would assume the item was tailor-made for her, it has around a 4% lower win rate than rushing Archangel's second. (Stats will be at the end of the post)
At that point, Diadem is basically a bait item on Sona, and good players shouldn't be building it.
The reason is pretty straightforward: enchanters, Sona included, don't actually need 1000 mana in stats.
A quick side note on Sona:
Sona is actually the one enchanter where the 1000 mana is convenient, because her mana costs are currently so high that she often wants to rush either Diadem or Archangel's second. But even in that situation, Archangel's is simply the better choice because it synergizes much better with Sona's AP ratios while also providing a valuable protective shield.
That doesn't really make Diadem's mana any more justified, though. If Sona genuinely needs a 1000-mana item as her second item just to keep up with her mana costs, I'd argue that her mana costs should be adjusted instead. She should be able to function with a more normal enchanter mana economy, while obviously taking her exceptional hyperscaling into account when balancing those costs.
For the other enchanters, mana regeneration already removes most mana issues starting from the first item, and by the time you reach your second item, it becomes extremely difficult to actually run out of mana in normal gameplay.
So a huge chunk of Diadem's mana is essentially figurative value. It's largely overkill, and too much of the item's gold value is being invested into a stat that its intended users don't really need.
I don't think the concept of the item is bad, though. The passive is interesting, and I like the idea of having Tear stacking eventually lead into a unique support item.
Instead of removing that identity, I'd consider changing what happens once Tear is completed: reduce the amount of value tied up in mana and invest more of the item's power directly into Diadem's passive.
That would preserve the Tear → stacking → transformation gameplay loop while making the actual finished item more attractive to a wider range of enchanters.
Right now, the item feels like it has a very interesting concept trapped behind a stat profile that is mostly unnecessary for the champions who are supposed to use it.
If the goal is to have Diadem be a real itemization choice rather than a Seraphine core build item, I think its power needs to be shifted away from raw mana and into the identity of Diadem itself.