Thanos ravaged the Marvel Universe when he acquired all of the Infinity Stones and erased half of all living things with the snap of his fingers with the Infinity Gauntlet. However, despite committing one of the most heinous acts ever, Adam Warlock actually made the Mad Titan a secret protector of an Infinity Stone, giving him one to ensure it didn't fall in the wrong hands.
After the events of the Infinity Gauntlet saga, Adam Warlock was determined to be mentally unfit to be a God and wield the Infinity Gauntlet by the Living Tribunal. Warlock would split up the Stones to a team called the Infinity Watch. First, he gave the Space Stone to Pip The Troll, allowing him to control the laws of space. Next, Moon Dragon received the Mind Stone, which made her psychic abilities even more powerful. Drax the Destroyer received the Power Stone, as Warlock said the strongman was already "almost power personified." The time gem was ultimately given to Gamora. Meanwhile, the Reality Stone was given to a caretaker whose identity was just known to Warlock, described as "someone who'd frown upon anyone attempting to gather all the jewels for their personal aggrandizement."
In Warlock Chronicles #2 by Jim Starlin, Tom Raney, Keith Williams, and Jack Morelli, Warlock encounters Enternity, who questions but understands why he gave the Infinity Stones to the different members of the Infinity Watch, save the Reality Stone. Eternity calls his choice "completely unacceptable," and blames Warlock for making such a poor judgment call. Warlock explains he fully understands the pros and cons of his choice. He decides to pass through a disruption in reality, where he finds the Guardian of the Reality Gem, Thanos.
Warlock decided to include Thanos as his secret member of the Infinity Watch, as he figured the Mad Titan would not be able to control the Realty Stone by himself. Still, giving the man who just used the Infinity Stones to wipe out half of reality is a bold move.
Surprisingly, things worked out for Warlock. Thanos would assist him in defeating cosmic entities such as his evil doppelganger, Magus, while Thanos managed to keep the Reality Stone after and out of the hands of other threats. Naturally, Thanos would eventually seek the power of the Infinity Stones again - but while he was a secret member of the Infinity Watch, the item was well-protected and safe. It was a bold choice for Adam Warlock to trust Thanos with the Infinity Stones but it paid off - although, it's doubtful he would make the same decision ever again.