Press L+Y+A to access the cheat screen. Then enter one of the phrases to activate the corresponding cheatcode:

View credits:
Enter CREDITS as a code.

Invincibility:
Enter PELOR asa a code.

Nightmare difficulty setting:
Enter MPS LABS as a code.

Unlimited Mystical Will:
Enter OBADHAI as a code.

10 Anti-Venom:
Enter SPINRAD as a code.

10 Berserk Brew:
Enter THOMAS as a code.

10 Fash Freeze:
Enter ESKO as a code.

10 Fire Bomb:
Enter WEBER as a code.

10 Fire Flask:
Enter BROPHY as a code.

10 Firey Oil:
Enter EHOFF as a code.

10 Globe Potion:
Enter WRIGHT as a code.

10 Insect Plague:
Enter DERISO as a code.

10 Keys:
Enter KEIDEL as a code.

10 Keys:
Enter SNODGRASS as a code.

10 Large Healing Potions:
Enter THOMPSON as a code.

10 Large Will Potions:
Enter GEE as a code.

10 Medium Potions of Will:
Enter LU as a code.

10 Potions of Haste:
Enter UHL as a code.

10 Pyrokins:
Enter SMITH as a code.

10 Rod of Destruction:
Enter AUSTIN as a code.

10 Rod of Fire:
Enter DELUCIA as a code.

10 Rod of Miracles:
Enter JARMAN as a code.

10 Rod of Missiles:
Enter MILLER as a code.

10 Rod of Reflection:
Enter WHITTAKE as a code.

10 Rod of Shadows:
Enter DINOLT as a code.

10 Thrown Axe of Ruin:
Enter RAMERO as a code.

10 Thrown Daggers of Stunning:
Enter BELL as a code.

10 Thrown Daggers:
Enter MOREL as a code.

10 Thrown Halcyon Hammer:
Enter PRASAD as a code.

10 Thrown Hammer:
Enter BRATHWAI as a code.

10 Thrown Viper Axe:
Enter FRAZIER as a code.

10 Thrown Viper Axe:
Enter HOWARD as a code.

10 Thuderstone:
Enter ELSON as a code.

10 Tome of Lessons:
Enter PAQUIN as a code.

10 Tome of the Apprentice:
Enter BILGER as a code.

10 Tome of the Teacher:
Enter MEFFORD as a code.

10 Tomes of the Master:
Enter SPANBURG as a code.

10 Warp Stones:
Enter HOPPENST as a code.

10 Holy Water:
Enter CRAWLEY as a code.

Disable all active cheats:
Enter UNBUFF as a code.