I would probably want a Norn Queen as LoW with stats like:
A Norn Queen is a single model armed with three pairs of massive scything talons and bio-plasmic scream. Only one of this model may be included in your army.
Remaining W: M S A
10-19+ 12'' 8 6
5-9 9'' 7 5
1-4 6'' 6 4
M WS BS S T W A Ld Sv+
12'' 2+ 3+ 8 8 19 6 10 3+
Weapon Range Type S AP D Abilites
Bio-plasmic Scream 18'' Assault D6 7 -4 1 -
Massive scything talons Melee Melee User -3 D6 You can re-roll hit rolls of 1
for this weapon. If the
bearer has more than pair
of massive scything talons,
it can make 1 additional
attack with this weapon
each time it fights.
A Norn Queen may replace any pair of massive scything talons for one item from Monstrous Bio-cannons or Monstrous Bio-weapons list.
This model may have toxin sacs and/or adrenal glands (see page 141. Tyranids Codex)
Shadows in the warp, Synapse. (page 85. Tyranids Codex)
Warp Field: a Norn Queen has a 3+ invulnerable save.
The Supreme Will of the Hive Mind: the range of your Shadows in the warp and Synapse abilities are 18'' instead of 12''. In additional add +1 to Psychic Tests in the Psychic phase.
Unending Hunger: At the beginning of each of your turn, roll a D6 for each friendly infantry unit within 6'', on a 4+ nothing happens, on a 1-3 it inflicts D3 mortal wounds and the Norn Queen regains one wound for each unit that had any models slain this way, on a roll of 4+. No more than 3 wounds can be regained per turn this way.
Once per battle it may devour a friendly unit/model that contain at least over half it starting numbers of wounds and regains D3 wounds, and the unit/model is slain. In additional it may replace one of its current weapons to that of the slain model had of your choice.
Weapon Beast: If this model does move in your Movement phase, it can shoot all of its weapons twice in your Shooting Phase.
Death Shriek: If this model is reduced to 0 wounds roll a dice before removing this model from the battlefield; on a 4+, it lashes out in its death shriek and each unit within 6'' suffers D3 mortal wounds.
Bio-tank: This model not suffer the penalty to its hit rolls for moving and firing Heavy weapons.
Psyker: A Norn Queen can attempt to manifest two psychic powers in each friendly Psychic phase, and attempt to deny two psychic power in each enemy Psychic phase. It Knows Smite power and two psychic powers from the Hive Mind discipline (page 121. Tyranids Codex).
Faction Keywords: Tyranids, Hive Fleet.
Keywords: Character, Monster, Psyker, Synapse, Norn Queen.
26 Power or 371points (Excluded Wargear Cost)
(Massive Scything Talons cost: 40 Points for 2 or more pairs)
I think this should be around a fair amount of power and/or points close to a knight, with a slightly lower wound count for a better invulnerable save and some regeneration, so either it would be focused down fast or slowly coming back to life during the game, with Adaptive Hunger ability it would be able to potentially devour some other monster to ''Evolve'' during the game, gaining a new weapon and making it more fun fluff wise as tyranids adapt to their enemies, I want some kind of ability like that during the game.
And by having a free option of weapons gear on a unique character rather than a certain specific ones, to personalize it a bit more, like custom building a knight that you can only have one per army, but it will match your own hive fleet with the weapon options that you can give it.
Shadows in the warp, Synapse & the Will of the Hive Mind are just the standard a Swarmlord and´/or Hive Tyrant has and Warp Field as it has all kind of genes inside it making sure its invulnerable save is slightly better than usual. It would probably be killed rather fast just like magnus or mortarion tends to die on turn one, by being focused down, knowing how most armies has tons of ways to deal mortal wounds in the lists.
Adding in Weapon Beast & Bio-tank onto a gigantic Tyranid close to the same size of a knight I felt like it was a must, while it has the ability to devour a friendly model/unit, like devouring a Tyrannofex for example gain a weapon like Acid Spray, its a very hefty price in points just to gain but it could add some fun fluff flavor and could save you in a pinch or screw you over completely.