Dagannoth sentinel is a monster encountered during the Blood Runs Deep quest.
Two of these powerful Dagannoth guard the entrance to the Dagannoth Mother's cave. They can use all 3 styles of combat: melee, ranged (the normal Dagannoth ranged attack) and a water magic spell.
While fighting, they can heal one another with "rallying roars", which show up in the text box as The sentinel gives a rallying cry and its companion's wounds heal slightly; this can make them very difficult to kill.
A good way of preventing this is to make sure they are both wounded equally. In other words try to wound one, then attack the second one until he ends up more wounded than the first one, then return to the first one and repeat until one of them is dead. After one of them has died, the remaining Sentinel will start attacking unpredictably with all three styles of combat. However, it should already be at low health and will be easy to kill. These monsters are also not immune to poison, so a good strategy is to use a poisoned dragon dagger at the beginning of the battle, special both monsters twice, and then use Balmung, switching back and forth between the two monsters.
Multiple Dagannoth sentinels are fight-able as a class B boss monster in the Dominion Tower minigame if the player has completed Blood Runs Deep. A good tactic here is to stand under one of the sentinels and attack the other. They don't heal each other if you do this and it can speed up the battle quite a bit. In freestyle mode, very quick afk melee exp can be gained by simply attacking only one and letting the other continuously heal. Over 200k melee exp an hour has been recorded with high strength bonus armour, Chaotic rapier with a Bandos Godsword for its defence-lowering special, overloads and turmoil. Void armour is recommended for this method for its 10% strength bonus.