Imperial Navy Grand Cruiser
The Grand Cruiser is a warship type used by both the Imperial Navy and the Fleets of Chaos. Smaller than a Battleship, a Grand Cruiser was still heavily armored and shielded, could still mount Battleship-grade weaponry, and yet still be faster and more agile than those great behemoths.
The designs of Grand Cruisers are ancient, with many of them predating the Great Crusade. With many of the complex systems that went into their construction now either lost or no longer easily replaced, the vast majority of Grand Cruisers have since been removed from active duty. Their place in Imperial fleets have instead been replaced by Battlecruisers, which are far easier to maintain.
Many however still serve in Reserve fleets, ready to return to front-line duty if the need is dire enough.
Like how the Endeavor-class and its variants are basically just one of the two weapon sections of a cruiser made into a ship, the Imperial grand cruiser classes are basically two of the three battleship weapon sections made into their own ship. Lego-tech is still proving its effectiveness.
Of course there are other Grand Cruiser classes out there, but those went over into the service of the Great Enemy. Those described below are the ones still known to be used by the Imperial Navy current era.
Grand Cruiser Classes
Basic
Vengeance
The class that the other Imperial Grand Cruisers are based on, the Vengeance was configured to take out its targets at extreme range. Armed with long-ranged Plasma macrocannon and heavy Lance broadsides as standard, the class has more than enough firepower to fend off a Cruiser or an Escort squadron.
Due to its lack of any prow and dorsal weaponry, the class is very vulnerable once any opponent manages to close the distance. It's particularly vulnerable to Attack Craft, as it doesn't have enough guns to defend itself from successive waves of attacks.
Carrier
Exorcist
A modified Vengeance with Launch Bays replacing the usual Lance batteries, the Exorcist was designed for extended operations alone at the edges of Imperial space, and is still considered the best long range patrol ship available. Rogue Traders in particular like its self-sufficiency.
Exorcists are also popular as a dual-purpose Colony Transport, where some of its flight bays are fitted to carry colonists and their equipment instead. Many colony convoys are thankful for the company of an Exorcist, as the Cruisers are able to outgun the raiders that would normally prey on such vulnerable vessels.
Ordinance-boat
Avenger
Armed entirely with short-ranged Macro batteries, the Avenger was designed as a linebreaker, pure and simple. The usual tactics involved rushing the ship full-speed into the heart of an enemy fleet, and cause as much damage and disruption as possible, wherein following ships would finish off the survivors.
Not many Avengers survive into the present-day Imperium, but those that do are looked upon fondly by both Navy personnel and the public-at-large as reliable and faithful warships.
In Battlefleet Gothic
Grand Cruisers are what one gets when a player needs the guns of a Battleship, but a bit tougher than a standard Battlecruiser, for just about the same point cost. Grand Cruisers are still just as fast and maneuverable as their smaller cousins, though with the lack of an Armored Prow they're much more vulnerable from attacks coming from the front, and obviously not as effective for Ramming.
The class does lack for a dedicated Lance-boat (in-universe, it's because said Lance-boat design went over to Chaos), so the Vengeance fills in for it somewhat, despite its macros having a longer range.
In Battlefleet Gothic: Armada 1 & 2
Much like on the tabletop, Grand Cruisers are cheaper (barring the Exorcist) and faster choice to fielding a Battleship. That said, the class suffers for the lack of variety, especially when compared to regular Cruisers.