When Thanos, surprisingly. However, the Mad Titan still made an appearance in this event, even if it was not in a form that fans would expect.
After Thanos' defeat at the end of Infinity Gauntlet, the Infinity Gems (later renamed Infinity Stones) are protected by Adam Warlock and his Infinity Watch for a time, then get stolen and taken to the Ultraverse, and finally make their return to the Marvel Universe. At that point, the Illuminati, a secret cabal of powerful heroes, collect the Infinity Stones and divide them among the group to keep them hidden and avoid future dangers. However, the Illuminati's secret dealings end up causing a huge new crisis. Parker Robbins, aka Hood, is a former street criminal risen to the status of supervillain after finding a demonic-possessed cape and pistols. During the events of Siege, Hood loses his powers and ends up in a prison for regular criminals, where he learns of the existence of the gems, and that one is hidden in the ruins of Attilan, the city of the Inhumans.
Robbins uses the wealth and connections he's gathered as a crime lord to reach that gem. Afterwards, Hood starts attacking the of the Illuminati to steal the other gems, which alerts the Avengers. Captain America is really angry at Iron Man for keeping the truth hidden, but they are still able to work together to try to stop Hood. In Avengers vol. 4 #12, by Brian Michael Bendis, John Romita Jr., and Dean White, the appearance of Thanos turns out to be fundamental to delaying Hood, but it is not, in fact, the real Mad Titan. To protect the final gem, the Soul Gem, Doctor Strange conjures an illusion of Thanos who tries to convince Hood that his mortal body could not withstand the power of the Infinity Gauntlet and that only Thanos is capable of wielding absolute power. The ruse works long enough to let the heroes arrive on the scene and take Hood by surprise, finally getting back the gems he's stolen.
The second Infinity Gauntlet saga is not as epic as the first one, which is still ed as one of the best Marvel stories of all time. Hood is certainly very different from Thanos, but in this difference lies the beauty of this story. While Thanos originally seeks the Infinity Stones to offer his beloved Mistress Death the gift of killing half the universe, Hood is only interested in his personal status and gain. He is a "regular" criminal who decides to live in the world of gods and titans, and he is aware he needs power to survive there. Thanos' motivations are grandiose and metaphysical, while Hood is driven by frustration and petty ambitions. However, both characters also have the same obsession and relentless lust for power, which makes them more similar than they look.
The choice of not having Thanos as the villain of this story is quite interesting. The second Infinity Gauntlet saga looks more grounded and realistic than the first one, mostly thanks to a relatable villain like Hood. However, Thanos will always be connected to the Infinity Gauntlet in the public's imagination, which explains why he still appears in this Avengers saga, despite not being actually there.
