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Why everyone is building Abyssal Mask on Cassiopeia, according to the numbers

By Renato "Shakarez" Perdigão
Aug 21, 2017
But it gives no AP...

In week 7 of the NA LCS, Keane brought to the rift a seemingly strange build for Cassiopeia, building Abyssal Mask instead of the much more popular and AP centric item, Rod of Ages. Was it a good decision?

Let’s look at the two items first. They both build out of Catalyst of Aeons, an extremely important item to Cassiopeia. Her constant spell slinging nature burns through her mana pool quickly, and catalyst’s’ eternity passive converts mana spent to health, while also converting damage taken into more mana.

Now let’s take a look at the base stats of both Abyssal Mask and Rod of Ages

Abyssal Mask grants:, 350 Health, 300 Mana, 55 Magic Resistance, 10% Cooldown Reduction, and a passive that increases damage dealt to nearby enemies by 10%

Rod of Ages on the other hand grants: 300 Health, 300 Mana, and 60 Ability Power.

Rod of ages does build more stats overtime, so after 10 minutes it will grant 500 health, 400 mana, and 100 Ability Power.

So the exchange for grabbing Abyssal Mask over Rod of Ages is sacrificing the AP and a bit of extra health and mana for cooldown reduction, an extra passive, and a ton of Magic Resistance. Typically, everyone’s most worried about giving up that AP.

So how does the 10% damage amplification on Abyssal’s Aura compare to the Ability Power from Rod of Ages?

Cassiopeia has fairly low AP ratios in her kit. This is to balance out the fact that she has a very low cooldown damage ability, Twin Fangs, and because two of her abilities are damage over time.

Noxious Blast - 23.33% AP
Miasma - 15% AP
Twin Fang - 10% AP
Twin Fang on Poisoned Target - 35% AP (40% on Patch 7.16)
Petrifying Gaze - 50% AP

Usually, Cassiopeia’s trading pattern involves hitting a target with Noxious Blast or Miasma to then close the gap with the movement speed buff from Q or the slow from W. Once she’s close enough, she unleashes multiple Twin Fangs, which will do extra damage as long as the target is poisoned.

It’s important to note that while Noxious Blast and Miasma have longer spell ranges, Twin Fang has a range of 700 units, which is exactly Abyssal Mask’s range. This means that most twin fang hits should be registering with the aura from the item’s passive.

Let’s look at TSM vs Dignitas from week 7. Keane finishes his Abyssal Mask at 12:33, after building Tear, Dark Seal and Corrupting potion.

At this point in the game, accounting for Bjergsen’s 38 magic resistance, each Twin Fang deals 201 damage if Bjergsen is poisoned. With the slightly cheaper Rod of Ages, Twin Fang would be dealing 204.

So, not too much of a difference. But Rod of Ages scales up to 100 AP. Very fair, let’s look at the two builds later in this game.

After ten minutes, at 22:33, the Rod of Ages would be fully stacked, and Bjergsen would have 40 Magic Resistance. Twin Fang on a poisoned target would do 265 damage, and with a mask would do 253, only 12 less damage.

Now let’s consider the value of the mask’s magic resistance. In both games that Keane built Abyssal Mask, he was facing a jungle/mid AP duo (Xerath-Sejuani vs TSM and Taliyah-Elise vs C9). This Magic Resistance is crucial to helping a scaling pick like Cassiopeia get through the early and mid game while she stacks her Tear.

For example, in the C9 game, both Contractz and Jensen itemize heavily for Magic penetration with Sorcerer’s Shoes and Liandry’s Torment.

At 23 minutes, Jensen has 43 Magic Penetration and Contractz has 34.5 Magic Penetration. If Keane had gone for Rod of Ages, he would only have 35.5 Magic Resistance, meaning Jensen would be doing True Damage and Contractz would be very close (Keane would only reduce 1%)

With Abyssal Mask, Keane has 90.5 Magic Resistance, which allows him to reduce 32% of Jensen’s damage and 36% of Contractz damage.

If Cassiopeia is facing a composition with very little magic damage, then Rod of Ages becomes the better purchase, as the extra health will help to deal with physical damage more than magic resistance ever would.

"I actually dont like that on the cassiopeia, banshee's veil would be a great item in that scenario" - Zirene

The argument for Banshee’s Veil is a valid one, but there are concerns regarding slot efficiency and item timings on Cassiopeia that make the purchase not as desireable.

Cassiopeia frequently goes for Archangel Staff, Rod of Ages, and Morellonomicon for 3 of her 6 items. If she wants to buy a Banshee’s Veil, she will be delaying either those essential starting items or a big 4th item like Void Staff. Because Rod of Ages is almost equivalent to Abyssal Mask, the mask is a great way to sneak Magic Resistance into the rotation without delaying a core item.

Abyssal Mask also grants 10% Cooldown Reduction, allowing Cassiopeia to reach the CDR cap earlier. With Morellonomicon, Abyssal, and Runes, she’s at the max 40%.

Abyssal Mask solves two problems in one, giving Cassiopeia access to a Magic Resistance item while not giving up on Catalyst, an important item for mana and health sustain, given her constant spell-slinging nature. It can be a very valid purchase when facing heavy AP compositions, and while it does result in a slightly lower damage build, the tradeoff is worth it given the damage mitigation that the 55 Magic Resistance will provide.

Renato "Shakarez" Perdigão profile
Renato "Shakarez" Perdigão
Shakarez has done work as a content manager for guide websites and also as a league analyst. As an enthusiast of good League, he usually rants about how teams are doing it wrong or instead talks about tank Karma being a great troll pick.
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