Eversource chief executive Joe Nolan called the site his company acquired "one of the most promising, multi-use interconnection points for large-scale renewable energy in New England."

The utility does not have specific plans yet for the property which housed what was once New England's largest fossil fuel-fired power plant; Eversource acquired it because of its critical location within the broader electricity grid. The company already has a substation complex next door, on the site that the Kraft Group now controls -- infrastructure that would remain after the soccer facility is built.

"We're trying to optimize what can be done for economic development with the Kraft Group and Wynn as well as ensuring ... clean energy interests are maintained for our customers," said Vandan Divatia, the lead Eversource executive on the deal.

Over the next few years, the Mystic 8 and 9 units will likely be dismantled; they had generated electricity from natural gas piped in from Constellation's nearby LNG terminal. In the meantime, Eversource will meet with state regulators, local officials and community groups to explore the best ways to reuse the site. It's in a Designated Port Area where new construction is essentially limited to marine industrial uses, such as power infrastructure. (The state Legislature recently removed the stadium parcel from the DPA.)

Executives at Eversource hope the property can accommodate new transmission connections -- underwater power lines in particular -- to bring in electricity from hydro, wind, solar or nuclear facilities elsewhere in the region.

"It does in some ways present itself as a Swiss army knife of an energy hub," Divatia said. "It ensures [grid] reliability. It has the potential of interconnecting resources which are pretty diverse."

Eversource officials said they're paying more for their Mystic land than Wynn did for the portion it bought in part because the section that was sold last year needs more environmental cleanup and came with easements for Eversource's existing equipment. The company said the transaction "will not have any immediate impacts to customer bills."

In 2023, Eversource and National Grid completed $49 million in grid upgrades to bring more electricity into the congested Boston area ahead of the Mystic plant's retirement. Divatia said those upgrades took care of immediate needs related to the Mystic plant's closure, but more work is necessary to properly prepare the grid, particularly if demand for power in Greater Boston rises in the coming years.

Danielle Burney, spokeswoman for the state Executive Office of Energy and Environmental Affairs, said the decommissioning of the Mystic plant is an important step forward for the state's clean energy transition, and that Governor Maura Healey's administration is excited for the site to be cleaned up and looks forward to working with Eversource and Everett officials to transform it to benefit the community.

Constellation issued a brief statement saying it's "pleased that Eversource is exploring site uses that would support electric reliability and the transition to clean energy." In Massachusetts, Constellation continues to operate the nearby LNG terminal, as well as power plants in Medway and Framingham.

The news of the sale generated a different kind of reaction at Everett City Hall, where Mayor Carlo DeMaria has championed the transformation of this section of his city into a dining and entertainment district anchored by the Wynn casino. DeMaria has already tried to oppose a nearby battery farm proposal, arguing that it's not in keeping with his vision for that area. DeMaria sounded a similar theme when his office was asked about the Eversource-Constellation deal, saying what he's learned about it raises "serious concerns" for the city.

"This appears to continue a pattern of burdening Everett with lower-value developments, such as additional battery storage, on a waterfront area with immense potential for higher-end commercial uses and vibrant recreational spaces for our residents," DeMaria said in a statement. "For a century, Everett has borne the costs of hosting energy infrastructure -- at the expense of public health and revenues."