Hello and welcome to A DIFFerent View. Today I will be talking about items that deal Magic Damage, and how to use these items to their best potential. The issue is a lot of items that give Magic Damage is that champions that may synergize with their stats and unique passives may not have any Magic Penetration, which can lead to the item not having very high gold efficiency.
First let's take a look at the items I am talking about:
Bilgewater Cutlass/Hextech Gunblade: These items have fallen out of popularity with nerfs to them. But at the same time Akali is growing in popularity so you may see this item more on her, which could potentially lead to people building it on more champions. Champions mostly use this item for the Lifesteal and Spell Vamp so not having Magic Pen is usually not a big deal.
Deathfire Grasp: This item is very popular and adds a lot of damage to a champions burst combo. Even if the champion doesn't have some kind of ideal set up that can allow them to use this item first in the combo (Veigar and LeBlanc), other champions such as Annie or Malphite have been building it. There isn't really an issue of champions without Magic Pen building this item, but if for some reason if you want to build it with 0 Magic Pen, I suggest you at least have the Ability Power to make it effective.
Executioner's Calling: This item isn't the strongest due to its poor item slot use late game. It still can have incredible effectiveness at top lane against some champions that are very dependent on their sustain. If you build this item it's likely early enough for the Magic Damage to still be effective, especially because the person you are laning against will likely prioritize Armor if anything.
Ionic Spark: This item isn't too popular either, but it can be helpful for jungling or split-pushing in some scenarios. The issue with these items that give Attack Speed and Magic Damage is most champions can't utilize both. Maybe a Shen or Teemo with at least the 10% Magic Pen from masteries can make use of it if they really want to split push.
Madred's Bloodrazor: This item has fallen out of popularity greatly, mostly because champions that can benefit from Attack Speed, Attack Damage, and Armor usually don't also enjoy Magic Damage. This item can be very helpful against high health teams, but it does cost a hefty investment of gold to build. If you think you may want to build it in a game consider taking the 10% Magic Pen mastery to help out with the damage, as high Health targets will likely have a good chunk of Magic Resist as well.
Malady: Like Executioner's Calling, this item is rather cheap and doesn't make the best use of an item late game. But it deals Magic Damage and lowers Magic Resist at the same time, making it somewhat efficient. If you plan on stacking other items like Madred's Bloodrazor and Wit's End, you probably want to include this in your on-hit build.
Sunfire Cape: I am really not the biggest fan of building this item with no Magic Penetration, you are basically paying 800 gold to deal 40 Magic Damage a second in a low radius area, so you better make that 40 damage worth it. If you want the Health and Armor, Randuin's Omen seems to be stronger, or even just two separate items that give both. However with items like Abyssal Scepter and perhaps other sources of Magic Penetration I can see strong potential for this item on melee tank champions.
Thornmail: This item doesn't always have the greatest effects if the enemy AD Carry has some Lifesteal and Magic Resist, which is why it's somewhat fallen out of popularity. It does give 100 Armor for only 2000 gold, but at the same time if people autoattacking you is an issue, than Randuin's Omen or even Frozen Heart can be more effective especially considering you will lower the damage output of enemy champions for your entire team rather than just yourself. With Magic Penetration this item can dish out a bit more damage than usual. But Sunfire Cape seems to be better at giving Armor and Damage.
Wit's End: Unlike the other Attack Speed + Magic Damage items, this item is very popular on bruisers. Mostly because it's so cheap you can buy it early game when enemies may have little amounts of Magic Resist, and then it gives enough Magic Resist to be worth keeping later in the game.
So of this list the more popular items are Deathfire Grasp and Wit's End. Deathfire Grasp makes perfect sense, it scales with Ability Power and champions that build it almost always have Magic Penetration, it's just overall a great item. Wit's End is powerful due to its low cost and the ability to give 50 Magic Resist to the user. Champions will usually buy it before at the enemy carry has invested in Magic Resist like Quicksilver Sash or Guardian Angel.
The Best Friend of these Items
There is a very common item that makes these other items a lot more viable in a game. That item is Abyssal Scepter. Abyssal Scepter is used a lot because it's such a great item. Mid lane champions love it because the AP, lowered Magic Resist, and bonus Magic Resist greatly increase their damage output while lowering the amount the enemy AP carry deals to them. It is also very strong in combination with some of these items on farmed tanks such as Shen, Malphite, or Amumu. The combination of Abyssal Scepter and Sunfire Cape gives great defensive value, while also a good amount of damage. But even if just an ally has Abyssal Scepter, if you think they are able to stick around in the fight for a while, you can consider it as Magic Penetration for you.
These items are okay, but try to make sure you have Magic Pen to make them efficient. The design concept of combining Magic Damage and Attack Speed is very interesting and it's meant to keep balance with these items. If Wit's End or Madred's Bloodrazor did physical damage they would likely require nerfs because the combination of those items with a late game Last Whisper on bruisers would be incredibly devastating. Thank you for reading and I hope this helps in your future games!