Unstable Portal

Unstable Portal is a rare mage spell card, from the Goblins vs Gnomes set.

Strategy
While the random minion generated is the most obvious determinant of Unstable Portal's value, the mana considerations of the spell itself also present both benefits and drawbacks. Players must weigh Unstable Portal's potential against these concerns, including the chance that it simply makes a "bad" minion that fails to synergize with the deck or is inapplicable to the situation.

The general function of this card is to pay 2 mana for a minion that costs 3 mana less, so you gain 1 mana overall. However, mana cost cannot be reduced below 0, so the 3 mana reduction will not always occur due to the number of minions in the game that cost 2 or less mana, reducing the statistically expected benefit. Among collectible cards as of Descent of Dragons, there are 252 2-cost minions, 137 1-cost minions, and 4 0-cost minions.
 * Mana savings

As a card draw effect that provides more mana savings than it costs, this card combines well with spell-synergies, allowing the mage to play twice as many cards for what will likely be 1 mana less in total. Examples include Mana Wyrm, Gadgetzan Auctioneer and Archmage Antonidas: with each of these cards, Unstable Portal triggers their effects for 2 mana that will be "refunded" later when the generated minion is cast at a discount. Furthermore, the mage gains the benefit of the spell synergy effects right away, even when the generated minion is best saved until later.
 * Spell synergy

The basic strategy for Unstable Portal is to take a possible loss in tempo - by spending 2 mana to play the spell - in exchange for a possibly larger tempo boost by virtue of the discounted minion. Depending on the minion generated, it may be possible for the card to "provide returns" right away, but usually the boost occurs in a subsequent turn.
 * Tempo

In the early game, it is often ideal when the generated minion costs one more than the total mana for the turn, allowing the minion to be played immediately, one turn earlier than normal, as with the Coin. For example, something like an Injured Blademaster (3 mana) generated on turn 2 can be played right away for a nice tempo boost. However, the earlier the turn, the higher the chance is that the drawn minion will not be able to be played until a later turn, meaning there will be a delay between the cost (and tempo loss) of this card and the hopefully worthwhile return.

Due to its random nature it can be a big risk to use this card early on, especially facing aggressive decks. "Losing mana" by getting an inexpensive minion like a Mana Wyrm, or losing tempo getting an unplayably expensive minion, could cause you to forfeit early control of the board, which can lead to a fast loss. This is therefore not a reliably strong early play, although it does have the potential to provide a significant advantage. If not under great pressure, the chance to be able to play (for instance) an 8-mana minion on turn 5 may outweigh the absence of a 2-mana minion on turn 2.

By providing a discount that lasts indefinitely (until the minion is played), Unstable Portal can increase "burst potential" late in the game by allowing more cards to be played in a single turn. This might be used in a damage combo, such as a 2-mana Leeroy Jenkins being played with other damage, but it could benefit any situation where the minion might normally be killed before its abilities can be combined with other cards, or "flooding the board" with many minions at once is desired.
 * One-turn combo

This advantage applies even to cheap cards that gain no net benefit from the 3 mana discount, potentially making them more of a worthwhile result than it initially appears. Even a Young Dragonhawk reduced to 0 mana cost makes it easier to immediately "power up" a Questing Adventurer cast on a later turn, despite there being an overall mana loss from the Unstable Portal and Dragonhawk combined.

Class cards from non-mage classes may be generated by Unstable Portal. This can allow for powerful synergies with mage cards that are usually impossible to achieve, for instance Prophet Velen doubling the power of the mage's numerous damage spells and hero power. Perhaps the most unexpected draw from this spell is Lord Jaraxxus: despite being a mage, Jaraxxus can be played and fully transforms the player into the alternate hero as per the card's usual behaviour.
 * Cross-class potential

The extreme randomness of this card makes it hard to strategize around. Even if it turns out that it would have produced the perfect minion for a given situation, cautious players may still choose to use a less perfect option in their hands since they can rely on those options to not be total disasters. This may leave Unstable Portal in the position of only being used when a player is either already winning, with mana to spare on a potentially "useless" minion, or when completely desperate and lacking other options. Some players may therefore prefer other cards which, even if more limited in potential than Unstable Portal, can at least be reliably compared with other lines of play to weigh potential outcomes.
 * Fundamental unpredictability

Lore
are a line of useful  spells that allow the caster to transport themselves and their party members to one of several fixed locations (including all friendly capital cities).

The art for Unstable Portal depicts a portal to Ironforge. This card shows the city's snowy exterior, while Ironforge Portal shows its molten heart.

Trivia

 * The artwork for this card comes from the World of Warcraft Trading Card Game "March of the Legion" series, for the card.
 * This card was hinted at by the first Innkeeper's Journal blog, Patron Dispenser, which revolved around mage portals bringing customers to the inn. It also featured the card art later revealed to be used in the Unstable Portal card.
 * Curse of Naxxramas ' Webspinner served to "set the stage" for the later introduction of Unstable Portal and the various piloted shredder minions in Goblins vs Gnomes, employing a similar design but with a far greater degree of RNG. Ben Brode states that players responded "very well" to Webspinner's random generate ability, and as a result the designers "doubled-down" the mechanic for Goblins vs Gnomes. The spell appears to have also inspired the five Portal spells added with One Night in Karazhan.
 * The location seen through the portal is the dwarven city of, making this a literal Ironforge Portal, although that card was not added until nearly 2 years later.
 * When Medivh is the mage player's hero, a voiceline will play, saying "And from the portal comes...!" presumably from a younger Medivh.

Patch changes

 * Arcane spell school added.
 * Added.