Hi dotacash community, here's a guide about iO the Guardian Wisp, probably one of the most unused/underestimated hero in DotA.
To get started, let's say already that wisp is a fake strength hero. Obviously, he's made to play like an intelligence hero. NO DPS ITEMS please.
SKILL BUILD
1-Tether
2-Spirits
3-Spirits
4-Overcharge
5-Spirits
6-Relocate
7-Spirits
8-Tether
9-Tether
10-Tether
11-Relocate
12-13-14-Overcharge
15-Stats
16-Relocate
17-25-Stats
Get tether first. Spirits are useless before level 3 (unless you are facing very very very paper heroes so level 2 spirits can deal some damage).
Get a level of overcharge. 40% IAS can help a carry at any time. But don't max it before tether/spirits : stun is more important in early game.
Before playing wisp, learn to use spirits (2nd skill). It's your bread and butter, if you can land your spirits correctly, you already damaged ennemy by 500 hp.
ITEM BUILD
1-Bottle
Always bottle, you are a peper hero and you need mana. Bottle gives it all to you AND to the ally you are tethered. Very cheap item. ALWAYS GET BOTTLE FIRST...
2-Power treads/Arcane boots.
The kind of boots you get depends on your team. If you got a very mana dependent team, grab arcane boots. If not, go STR treads, it will give you some bonus hp you need. You do not need mana boots only for wisp, bottle can do the job easily.
3-Mekansm
Best item ever for wisp. You can heal any of your allies in difficulties by tethering them. If you are at least 250hp damaged, your ally will get +500 hp.
4-Pipe/HH/BKB
Now, we are at the point where you have to chose your item depending on the game.
If you face a lot of nukes/aoe... go pipe
Pipe gives your team barrier and make you more resistant to magic damage, which helps a lot.
If you face DPS heroes... go HH
Why not blade mail ? Because wisp can escape easily with tether and the stun it provides + mekansm healing. Heaven's halberd makes the ennemy DPS fail whil it also gives you lot of hp and 25% evasion. Makes wisp look more ninja.
If you face massive disable team... go BKB
BKB is an item you won't get often, but in those games where there is massive try hards pick, it can be very usefull. BKB will give you the opportunity to dodge ennemies disables, but also the possibility to help your team survive by stunning with tether.
5-Heart
If you can reach this late in game, get heart. Very good for escaping, you just turn overcharge on and it heals anyone by 2% of your hp per second.
DON'T GET THOSE ITEMS...
Shivas: You don't need mana, give shiva's to another intel hero that needs it way more than you.
DPS items: You are a wisp.
Dagon: You support, why would you get dagon.
GAMEPLAY
First of all, best to to maximize wisp utility is to pick a good combo.
GOOD combos are : riki / ck / pa / xin / ursa / ...
Any carry that can blink is the best shit ever for a wisp combo. Why ? blink makes tether to autostun, and your 70% IAS from overcharge is way better on a riki than on a leshrac.
Tether :
Best way to use tether is to stay in the fog near an allied hero (within 1800 range). That way, the ennemy cannot see you and you can dive at any time by tethering your ally or any allied unit. Use tether to heal mana or hp to your allies by using bottle, mekansm or arcane boots. Use it also to stun ennemies in fight, do not tether your ally and just send your poor missile that just tickles a bit. Run around and try to stun ennemies (+ damaging them with spirits).
Spirits :
Before playing wisp, learn how to use spirits, those can be very deadly especially early to mid game. In team fights, the 300 AOE of explosion makes you able to remove 500 hp of the whole ennemy team within 2 seconds. Spirits can also be used to creep when at lvl 3-4, they do some cool damage. But you are a support, so it is luxury to creep/farm. When you tp anywhere in the map (by using tp or ultimate), always have spirits fully deployed so you can make damage as soon as you get in the fight.
Overcharge :
Very good spell used mainly to boost your carry or heal someone. 70% IAS and 20% damage reduction is very good when you give it to a hard carry early/mid/late game. It makes the carry look 3-4 levels ahead. Also, when you are tethered to a unit because you want to heal it, it is important to use overcharge if you are full hp/mana. It will not heal anything if your are full, you will just waste bottle/mek
It's very important to activate overcharge everytime it is needed, every second counts in a fight! And do not forget that this spell has no cooldown, so you can abuse it.
Relocate:
This is the ultimate ninja skill of DotA. It makes you able to go anywhere explored on the map to gank or save someone. If you ABSOLUTELY have to gank someone or there is a gank that you cannot miss to put the game in the bag, use relocate with tether to bring anyone with you (CARRY PLEASE). You can bring naix in you too for a triple tp

... If there is no gank possible or the chance of getting the ennemy hero within 14 seconds (relocate duration) is not 100%, do not use relocate to gank, use it to go back to base full your bottle, your mana, your allies mana/hp, and bring the items you need. Relocate does not have a big cooldown, it is 90 seconds by level 1 (75 seconds after tp) and go down to 60 seconds (45 seconds after tp) at level 3. It can be very good to use this spell to save someone from a certain death by tping him into you're fountain too. Don't forget that tether gives you 20% move speed so you can cross the map pretty fast if you need to and relocate is on CD.
Do not forget that wisp is a support hero !!! Using him in any other way means you are playing vs bad pubs or that you don't really care about winning the game

Always try to lane with a carry, you are probably one of the best babysitters in the game. NEVER SOLO MID, what a waste of a lane for wisp.
I hope this guide will help some people that wanna try wisp or other people that wants to master it !!!
If you have any questions, PM me!