The Xerath Nami botlane is an idea we were playing w my friend for the last few months since I am a Xerath main and he is a Nami main. We have been playing this for really long time and its really nice lane bully if you get experienced at it. It´s rare for us to be losing in lane. It is a lot of fun and also very effective strategy at the time.

This botlane combo is working so well because Nami is the only enchanter who is able to directly apply effects on her teammate´s abilities and is very usefull at buffing mages. With Xerath you can utilize Nami E from very far away with the imperial buff.

Imperial on Nami:
Nami has one the best (if not the best) synergies with Imperial Mandate in the whole game. She can utilize it in any scenario and is always the best item, you can even rush it before mythic item. When Nami gets this item, your damage in lane spikes so hard that its disgusting.

From the Xerath´s side your way to utilize the Nami´s imperial is to take some after effect which will trigger the second part of passive to instantly deal bonus magic damage and the movement speed.

The after effects could be:
- Arcane comet
- First strike
- Liandry´s anguish

Your teamwork in this combo is very important! You have to play almost like a one player. It´s really useful to hear each other on Discord or something I´ll write some important points for both of the players:

Nami:
- In laning phase, E Xerath everytime you see he is charging Q/R or if he tells you
he´s going to trade, your E is the key to winning trades
- Don´t be afraid to go and trade from closer range than Xerath will do, he has much longer range than you
- In teamfights, play as you would normally, just your E should always go to Xerath, so you have to always follow him in lategame and buff his spells.

Xerath:
- Do not forget to tell your duo when you are going to trade so he will E you
- Nami with Imperial is really good at stealing kills by accident with W or Aery
- When you have rank 2 ultimate and are ahead (2-3 items), you should be able to kill squishies only by ult with Nami E.

Your earlygame is not so bad if you aren´t playing into some earlygame aggresive lane (Pyke, Samira, Draven, Leona etc.). If you are good at hitting spells as Xerath and good at trading with Nami,. Into some botlanes which are weak early you can really give them a hard time by the poke, outtrades and even some 2v2 kills. Into some matchups you can´t play too aggresive, try to just poke them with Xerath with help of the Nami E.

When you both get one item, you powerspike disgustingly. You have to take advantage of that. You should be able to make the enemy adc back after 3-4 buffed Xer Qs. From this point you are very strong to the end of the game unless you get erased by an asassin.

Nami:
The key to Nami being so good in this combo is Imperial Mandate. This item´s passive is really usefull on Nami in buffing not just Xerath, but most of caster champions. In this combo, you should rush it as first item even before you mythic item.

Mythics viable are Shurelya´s Battlesong or the new mythic Echoes of Helia. These two items are pretty optional and you can choose which one of these two you want more.

Antiheal is good on Nami because you can proc the 40% antiheal through your allies, that means when you build antiheal, the champion you heal also gains the antiheal.

Xerath:
Regarding the previous part, you should never build antiheal. Nami should always build it so you can buy more damage items.

There is nothing really interesting in mythic itemization. You build Luden´s when you play against squishies, Liandry against tanks and Crown if you want to survive burst.

I would avoid Archangel´s Staff, I don´t think the item is better than (for example) Shadowflame, Void Staff or Horizon Focus.

In short, you can itemize whatever you feel is right.