This is a meme that certifiably works at least at 2100 MMR and below. It hasn't been proven at higher MMRs. Yet.

The core premise is that the same patch that removed permanent Atrophy Aura damage gain (from enemy hero deaths) buffed the TEMPORARY damage gain (from creep deaths) in both strength (5 to 8 over two patches) and duration (60 to 75), and the temporary damage has a really fun synergy with Battle Fury.. The faster you can kill creeps, the stronger your right click gets, which further accelerates your farm speed (with battlefury's cleave). And if you can switch from farming to fighting quickly then you can hit enemies with your massively boosted (e.g. +200) right click. But Underlord is slow. So I buy Travels, which also happens to suit Underlord's potential as an aggressive teleporting pusher (on top of his ult). So you can farm at high speed and then teleport towards fights and obliterate people with massive cleave right click. But these two items (BF > Travels) leave me a bit fragile, so next I buy Heart, and at that point I'm suddenly obscenely strong (by 2000 MMR standards).

A notable weakness of P1 Underlord is that if you die and the enemy is still forcing a fight when you respawn, you'll be weakened by the fact that you don't have any Atrophy Aura damage built up. So it's possible to lose a lot of momentum. But buying Heart has so far made this much less of a problem, because it's a lot harder to die and a lot easier to just keep your momentum going when you're that tanky. Watch out for stuff like Spirit Vessel though (e.g. regen-reducing stuff and percent-based damage).

Your ult lets you push out lanes riskily similar to Jugg's spin-TP pushing, and you should absolutely do this, because you want to be playing aggressively.

The big timings are
1. Battlefury (around 11 minutes). At this point you should usually tp to offlane so you can farm ancients.
2. Travels (around 15-16 minutes). At this point you can start participating in fights more and potentially taking riskier farm (using your ult so you can overextend a bit and escape easily.)
3. Heart of Tarasque (around 22 minutes). So far (in three games) this has proven to be a massive power that lets me get away with ridiculous things because I'm such an intense mix of durability, mobility, AOE zone control, and immensely strong right click. And I'm generally upleveled just from fast farming. You can invade the enemy base at this point if you're careful. You don't necessarily need a Rosh. (Rosh can get a bit awkward since if it takes too long then you lose Atrophy Aura damage so you're not in peak condition if the enemy team interrupts your Rosh attempt.)

Once you have Heart you can be mostly concerned with pushing the enemy back into their base and warding/farming their jungle and then invading their base, but your next core item is typically S&Y for the great regen amp (on heart) and the further movespeed boost. But sometimes you may want Blink or perhaps BKB.

Draft-wise, I recommend always banning Jugg and only playing P1 Underlord is Jugg is banned unless your team has some kind of great Jugg counter with something that can save you from Omnislash. Jugg is a bad matchup for Underlord in general; he can spin through your AOE zones and you don't have a built in answer to Omnislash and you don't really want to build one.

Aside from that, Underlord likes allies with long-range slows/stuns (since he's not very fast but wants to right click people) and also likes allies with dispels, or Omniknight (sweet 50% status resist and massive strength buff).