CRIMSON COBALT Build: Marks and Attack Combos
Use CRIMSON COBALT in Patch 26.21: land the designated spell, attack the marked champion and build around reliable follow-up rather than assumed infinite resets.
In Patch 26.21, 
CRIMSON COBALT has two steps: damage an enemy champion with the designated ability, then attack that marked target. The attack deals bonus magic damage and reduces that ability’s cooldown. Read which ability the card specifies; casting any spell before an attack is not enough.
Mark and attack the same target
The mark lasts 4 seconds. Follow up before it expires. In lane, a target moving up for a minion can offer an opening. In fights, mark someone you can reach rather than a backline target while you are stuck hitting the front.
If it does not trigger, check the designated spell, the mark on your attack target and whether the mark expired. Long-range spell damage alone offers little cooldown value if you cannot follow with an attack.
Choose champions that already weave attacks
Hecarim and 
Fizz are examples of champions that approach enemies and mix attacks with spells. The designated ability still needs to land reliably, with room for another rotation afterward. A fast cooldown does not help if you are immediately locked down.
Build for the champion’s mana, haste and survival needs first. Bonus magic damage does not mean every AD champion should switch to ability-power items or drop their usual core purchases.
Pairing it with Ethereal Weapon
Ethereal Weapon lets abilities apply on-hit effects, making it a common pairing to consider. Whether it replaces a manual attack depends on the specific ability and interaction. Do not assume universal infinite resets; use the basic spell-then-attack sequence until the interaction is confirmed.
FAQ
Does CRIMSON COBALT enhance every ability?
No. It applies to the ability named on the card. Check your in-game version before judging how reliably you can mark and attack.
Can I attack anyone to reduce the cooldown?
Attack the marked enemy champion. Hitting one champion with a spell and then attacking a different, unmarked target does not complete that sequence.
Can I cast again immediately after the attack?
Check the cooldown remaining after the trigger. A reduction is not necessarily a full reset. Wait for the ability to become available, and avoid canceling an attack just to force another cast.