Gren may activate Unhallowed Crypt with 0 ranks in it.
Attack speed Marks and Quintessences are recommended.
Gren is a jungler with an odd form of sustain. At the start of the game you will want to place your crypts near various lanes hidden away in hard to find alcoves or near turrets. It has no mana cost before 2 minutes so take the time to set them up.
When ganking, make sure to stop by a crypt to regain some health. Use Rigor Mortis' active to get as close as you can. Don't be afraid to use if on an enemy minion as you only need to get close enough to activate Clawed Grip.
Only use Startling Intelligence while clearing jungle camps. Rigor mortis' cooldown is too large to waist, and the health cost of Clawed Grip isn't worth wasting on Monsters.
Startling Intelligence is your source of AOE crowd control, but as fights go on it turns into your largest source of damage instead. You will only be able to CC them at the start, but as the fight goes on Desecrating Presence will eliminate the enemies ability to damage you and your team.
If you aren't near a Crypt, leap in with Rigor Mortis and begin fighting. Clawed Grip will bring the carries to you, or force them to back off for a time being. Desecrating Presence takes some time to ramp up. Don't get too eager to apply it, hoping in too early will still get you killed.
Goulish Vengence is a terrifying ability. Enemies will often refuse to fight near Crypts because of this. Remember that during fights the only way to get into a crypt will be with Rigor Mortis. Do not waist it to engage if you think they will do so for you.
A combination of health and resistances is the optimum build on Gren. You will want to pick up some AP/Tank items as you will need to deal some damage in order to get the enemies attention. Sunfire Cape and Iceborn Gauntlet are great picks as they can get Desecrating Presence started and deal some nice damage while offering greater defence.