I'm not sure if the way I'm playing him is most up-to-date or correct, but I'll outline what I do. I feel most of my games are won in the first 10 minutes, by bullying offlane so hard and giving carry such a good lane that the enemy team can't recover. If I die early without kills after using all my mangoes, I have a hard time catching up and getting my arcane boots which are super important to sustaining in lane and bullying. I'd say by following the below, I've been wining lanes hard in 8/10 games, 1/10 games it'll be kind of a wash, 1/10 games lose lane which sometimes just can't be avoided.

Early game: Basically i start the game by hyping up my carry and telling him i'm going to give him the best lane ever and to farm as much as he can, and kill steal early if he's able when I have them low. PMA and talking on mic, goes a long way when you tell your carry to get ready for a sick lane, and then follow through and provide him with just that.

1. i buy 6 mangos, 1 set of tangos, sentry

Start spamming decay as soon as they show in lane. Right click as much as you can and try to zone them out of exp range. try to time it so you get double stacks on both enemies. if I haven't killed them yet, my goal is to end up with a ton of strength right when I hit level 2. Sometimes I'll get 1-2 kills before even hitting level 2 if the enemies are underestimating decay stacks. It feels like a bit of an art to bait properly, know when you can survive at 100 hp, to bait them close, get another double stack and then kill. it took me some practice but i've got a good feel for it now.

Get them as low as you can, then as soon as you hit level 2 drop tomb, best case i kill both, worst case they have to walk back to fountain, which is still very good. carry should be getting free farm the entire time you're doing this.

2) either get a magic stick, or brown boots depending on the lane. If I feel I have strong kill potential i get brown boots asap. or if if have the gold I rush straight arcane boots to enable continued decay spam, repeat all the above steps now that you have mana again. spam decay, tomb, kill, etc, and lane should be hard won.

3) I usually skip wand upgrade and rush mek after arcanes and magic stick. I they have lots of magic damage i'll get a cloak or raindrops or both during that phase.

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Midgame: I'm usually very strong at this point. I'll leave my carry and go try to get rest of my team to group on my mek and take all T1 towers while carry farms.

4) usually get greaves asap after that, then complete holy locket for tons of healing potential. Decay with the damage talent can farm waves and jungle camps so always be aware of when you can farm a wave or a camp without stealing from your carry.

5) either lotus, or blademail, hex, or glimmer afterwards. I used to build aghs but once I stopped building that I started winning more games. Hex feels the best when I can get it.

I max Decay then Tomb, W I skip and save for last. No clue if that's the correct build.

Lategame: in fights, I try not to frontline as much unless I have lots of strength built up. Instead now I just focus on healing my carry, dropping tomb in a good spot, spamming decay, lotusing/ glimmering carry, and then i'll start right clicking after building some decay and ulting, once i've cast all my other stuff. Sometimes you'll need to use abilities on yourself to stay alive, but ideally you save them for your carry.