Spear of Konor campaign: Both Legions bring out the Big guns

Desperate to turn the tide of battle on the Spear of Konor to their advantage, the Iron Warriors' 31st Grand Company deploy a 5-strong Tyrant Siege terminator team, along with 2 Contemptor Dreadnoughts and 3 tactical teams (including one Breacher squad), as well as a 6-strong Cataphractii Terminator squad in a special mission to capture and cripple some of the cooling systems for the Spear's main lance cannon.

The Tyrant Siege Terminators take up position at the end of one of the main corridors leading onto the one of the hall's where the Ultramarines are guarding are guarding the cooling systems

However, the Ultramarines had prepared for just such an attack and guarding the cooling tanks was a similarly strong force, but with a Deredeo Dreadnought with Anvillus Autocannon battery as well as an Aiolos misslie launcher.  If attacked, a Breacher squad would attempt to hold the position sufficiently long for reinforcements to arrive: a third tactical squad, a Suzerain Invictarii squad and a 6-strong Cataphractii squad.

No way past the firing arcs of the Ultramarines' Deredeo Dreadnought, Kratos the Destroyer

The Iron Warriors attacked the cooling tanks from one side, sending a Breacher squad up the Middle to establish a shield wall behind which other units could advance and then reinforce either of the other attacks.  On one side, the Tyrant Siege Terminator squad took up position at the end of a long corridor leading to one of the halls containing the one of the cooling tanks.  On the other side, a 15-strong tactical squad advanced swifty on the position guarded by the XIIIth Legion's Deredeo Dreadnought, hoping to draw attention away from the other advances.

An Iron Warriors' Breacher squad advances quickly up the central corridor, looking to establish a defensive position protecting a route to both cooling systems

Not all went as planned for the IVth Legion, however.  The Ultramarines' Breacher squad were able to stand firm under the salvoes from the Tyrant Siege Terminators despite heavy casualties, and their graviton guns reduced the Tyrant Siege terminators to just two, allowing reinforcements in the shape of a Contemptor Dreadnought, Agrippus the Interred, as well as a unit of terminators, to arrive to secure the position.  On the other side, the Iron Warriors' tactical squad was ambushed by a 15-strong Ultramarines unit, and a brief firefight ensued before the two units engaged in hand-to-hand fighting that lasted for the remainder of the engagement.

The Ultramarines keep a strong hold on the right, but are under huge pressure on the left

The XIIIth Legion's Bracher Squad hold firm under a relentless bombardment from the Iron Warriors' Tyrant Siege Terminators.

Meanwhile, the XIIIth Legion's Deredeo dreadnought was able to slow the Iron Warriors' advance; his aiolos missile launcher cutting down the first line of an Iron Warriors' tactical squad, and his autocannon battery wounding the vanguard Contemptor Dreadnought, Ferrox the Relentless.  When the Ultramarines' Cataphractii charged the Iron Warriors' Breachers, it quickly became apparent that the IVth Legion would not capture the cooling towers, and the Iron Warriors were forced to retreat.

The Iron Warriors' Dreadnought, Ferrox the Relentless draws away the Kratos the Destroyer, allowing the IVth Legion's Cataphractii terminators  to advance

The Ultramarines' Cataphractii close in for the kill

The Ultramarines survive the Iron Warriors' onslaught, largely thanks to the bravery of their Breacher squad and the threat posed by the Deredeo Dreadnought.