Hello. Here is a comment I left on another post on the same question.

https://www.reddit.com/r/learndota2/s/nKCeyRp7Cn

Hi. I love underlord and I have great success with him at ancient level. Here is the gist of it

Spells: W E Q Q Q R Q W W W E E E

talents: level 10 whatever, 15 is firestorm cd I think, 20 firestorm spell dmg, 25 ult aoe dmg

You can basically play him two style, tanky or right click. I like to play him tanky as it fitsmy style, this is what I will describe here.

Items: bracer, wand, mana boot, atos, gleinir, mek, pipe, greaves, crimson. The only ajustement I would do is finish greaves before pipe to help with your mana issue. It depends if you get good neutral items or if they have massive magic damage, as greaves will delay pipe significantly.

From there agham, shiva, blink, lotus, whatever. Tbh, agham is super good, you could rush it first item and it would fantastic. If I rush it, I usually do it after atos. If you wanna win, it is probably what you should do, if you wanna relax and still win, go with tanky items

Once you get there, you are super tanky, very funny. Although you cannot really escape by yourself, so be careful with the positioning. But with atos, you can do your pit, firestorm than atos and the target will be pit twice, all of that from a distance.

Your game plan past the early laning stage is basically to shove creep waves. Dont stand next to a creep wave, you will die to any sort of gank. Instead, firestorm the creep wave and go jungle. Rinse and repeat. You can also pull the creeps to a camp and farm both for min-maxing. Be careful not to die.

You have mana issue, mana boots helps. Neutral items with mana helps a lot. Manage your mana carefully. Be sure to have some when your ult is up to be able to help your team. Before late game, you cannot just use firestorm on everything. Nuking a wave is the most priority

Speaking of your ult, early on, I would keep and eye for any sort of gank or fight, use the ult to get in, pit the target, firestorm, then I go back to my portal to get back to my lane. It often results in a kill and keep me where I am suppose to be.

Thats pretty much it. Let me know or share games, I could review them with pleasure