Io

Io the Guardian Wisp is a highly unique ranged strength hero who works best as part of a communicative team. With its support-oriented skill set, Io is able to share its strength through Tether and Overcharge, harass enemy heroes out of the lane with Spirits, and materialize itself and an ally anywhere on the map with Relocate. Gank enemy Heroes, push undefended lanes, save teammates from deep within enemy territory - the possibilities are endless, and Io features one of the highest skill ceilings in the game.

Tips

 * Tether can be used on allied heroes to get to their side with great speed, as it latches from a considerable distance.
 * Approaching a confrontation from behind will allow for easier use of Tether's slow. Positional items are also useful for this.
 * Spirits requires much practice and focus to control, and due to its high mana cost it is not advisable to use it frivolously.
 * Building Heart of Tarrasque or other HP regen items will allow for much more liberal use of your Overcharge. Also, the regen you get from these items will heal your tether partner, allowing you to heal allies by using Overcharge.
 * Remember that Tether can be used on friendly creeps, and in a dangerous situation Io may be able to avoid death by Tethering to a lane creep, owing to Tether's large latch distance.
 * Should an emergency arise, Io may Tether to a wounded ally and then Relocate to the fountain to heal both Io and his ally.
 * You can also save the life of a crucial allied hero with this method, but it will often result in your own inevitable death when Relocate forces you to return to your original location. Be sure to un-Tether your ally before you return.
 * The inverse of this can also work; Relocate from your fountain to the crucial ally and Tether them to bring them back. This is not always viable however, as it entails the 12 second discrepancy between Relocations.
 * If Io's team has an end-game carry such as Phantom Assassin or Faceless Void, his top priority becomes ensuring the survival of this hero at any cost, including his own life. Io's skillset makes him very good at doing this, and if he succeeds in preserving the life of his carry then they will eventually become hugely powerful, and most likely win the game for their team.
 * A good way to make Io cause a lot of damage with his nuke quickly is to summon the spirits, make them get closer, and when the cooldown has ended, get close to the enemy and activate the spell again. This way, all the spirits around you will explode damaging the enemy and five new ones will appear, potentially exploding on him too. Best used if Io is laning with someone with a good slow or stun, and when used to maximum potential (exploding all 10 spirits on maximum level of skill) will cause 1000 magical damage.
 * When ganking or escaping, always remember that Tether duration is the same as its cooldown, so it can be maintained on constantly as long as you have enough mana.
 * Though using Tether to offer bonus healing from items such as Tranquil Boots or an Urn of Shadows charge is a good idea, your Tethered ally will not gain anything if you are fully healed. Using Overcharge to lower your health and mana before using any healing items will assure you are able to pass along the boosted regeneration.
 * Soul Ring is an excellent Item if you have Tranquil Boots so that you can give your Tethered target HP regen and also the Soulring Activation Mana.
 * A Bottle is good for the same reasons as Soul Ring, Heal yourself and your partner in battle and give mana back too.
 * Necronomicon is an amazingly underused item on Io that is extremely powerful on him. Simply tether either the melee or ranged necro creep, depending on the situation, and relocate to gank a vulnerable enemy. It is also useful for pushing towers, and this can also be done using relocate. The Strength and Intelligence boost it gives also synergizes with Io's skill set and low base HP and mana.
 * Mask of Madness is a great situational item for increasing Io's DPS and survivability. Thanks to Overcharge, at max level Io only takes 10% extra damage for up to 170 IAS, 30% extra move speed and 20% life steal. Due to Io's fragility, however, it is important that Mask of Madness is used in conjunction with HP and Armour boosting Items, as he can't be burst damaged down while under its effects. Note that if you use Overcharge and Mask of Madness while tethered to an ally or Necro warrior/archer, the movement speed boost is increased to 47% for yourself, and the Tethered unit still gains the 20% damage reduction and 70 attack and 17% move speed speed bonus.
 * Drum of Endurance is a great pickup on Io, giving you cost efficient stats and further boosting you and you're tethered partner's attack and move speed. It also Stacks with the Necronomicon Archer's Movement and Attack Speed Aura.

Trivia

 * Io is the name of a priestess of Hera in Greek Mythology. She was seduced by Zeus and subsequently incurred Hera's wrath.
 * Io is also the name of one of four Galilean moons of Jupiter, which was named after the same character in Greek Mythology.
 * Wisp is the name of an Elemental from the Legend of Mana series, which has a similar appearance and is usually involved in healing.
 * Io's fun names in DotA were Weird Ball and Santelmo. Santelmo may refer to St. Elmo's Fire, a weather phenomenon that creates sparks of bright energy similar to that of lightning, mostly occuring from sharply pointed structures.
 * Enigma, Io, Keeper of the Light and Chaos Knight together represent the four fundamental forces of the universe with Io representing the electromagnetic force, as stated by Enigma Your charge goes out. This is reflected in several of Enigma's responses to these heroes.
 * Oddly enough, Io is referred in the bio as "it", but the ingame description for Spirits says: "Io twists the particles of the universe with his unimaginable capabilities."