Heavy split-push and healing.
Durability and crowd control
Every 4s your next attack on a champion deals bonus damage, heals you, and permanently increases your health.
Charge up a powerful attack against a tower while near it.
After 10 minutes gain 8 Armor and 8 Magic Resist and increase your total Armor and Magic Resist by 5%.
Heals and shields are 5% stronger and increased by an additional 10% on targets below 40% health.
15-135 health (at levels 1-18) and +9% attack speed
Takedowns on enemies grant permanent Attack Speed.
Takedowns restore health and grant bonus gold.
Trundle is one of the best users of Grasp thanks to his innate auto-attack based kit and stickyness with [Q] and [E].
If left alone Trundle is one of the fastest split-pushers. [Q] + Demolish means you'll be cutting down towers in no time.
If you're looking to teamfight rather than split-push, take Font of Life instead.
Conditioning's bonus Armor and Magic Resist make you a massive front-line tank in mid to late-game teamfights, especially where paired with the massive healing from your passive and [R].
Revitalize synergizes incredibly well with Trundle's natural healing from his passive and [R]. It also synergizes well with Font of Life and Overheal.
Trundle could always use more auto-attacks and this is your only source of Attack Speed.
If you're intending to build a lot of AS (like BotrK) you could take Legend: Tenacity instead.
Triumph empowers your ability to absorb massive amounts of damage in teamfights with your ability to pick up kills/assists and heal form your passive + Triumph.